processor - dual core or quad core for ad/ss

Neither AD nor DNS are particularly CPU heavy and for the number of clients you're talking about you could easily get away with a dual-core chip, even at a lower speed than you're suggesting.

MSQL will use all the cores you can throw at it pretty well, go for the quad core - oh and consider getting a dual-CPU E55xx series server for your DB, even if you just start with a single CPU today, moving to a second is so quick and cheap that you'll be able to expand you system very easily if you go that way.

For the AD/DNS server for that few users, either will more than do.

For the SQL server, then I would go for the quad core [unless it will only be running single queries most of the time (i.e. not large multi-user systems) in which case the higher speed of each core might be more beneficial than the extra cores] all other things being equal. "All other things" being mainly RAM (you want as much as you can shove in there) and IO (a good IO sub-system is a must for an active SQL server).

Can someone enlighten me as to which I should prefer based on the following uses?

Active Directory/DNS Server (<50 users):

1. Dual Core Intel® Xeon® E3120, 3.16 GHz, 6MB Cache, 1333MHz FSB
2. Quad Core Intel® Xeon® X3220, 2.40GHz, 2x4M Cache, 1066MHz FSB

For AD, I'd say Dual core. That's fine. Like others have said, AD/DNS is not CPU intensive. Don't sweat it. Personally, even these CPUs are overkill for an AD/DNS setup for 50 users. Now, if the number of users increases to say.. 500? 5000? These would be better suited. Maybe you can find a cheaper DC/QC CPU for the AD/DNS server?

Sql Server 2008 DB Server:

1. Dual Core Intel® Xeon® E3120, 3.16 GHz, 6MB Cache, 1333MHz FSB
2. Quad Core Intel® Xeon® X3220, 2.40GHz, 2x4M Cache, 1066MHz FSB

I'd go for the Quad core here, not for the slower FSB, but rather larger cache. However, #1 is 'wolfdale' 45nm CPU (~65W) but #2 is 'kentsfield' 65nm CPU (~95W). Someone could argue that #1 might be a better choice for the DB server as its slightly higher FSB and more L2 cache per core that make #1 an interesting choice over #2. I'm sure SQL 2008 is optimized for whatever resources can be given, but in general database servers can usually use every possible resource in a system if conditions and I/O become issues. I'll address this specific I/O issue later..

From an energy perspective (which you may/may not have interest in), the 'wolfdale' (#1) CPU in comparison to #2 is a better choice.

fyi - the price point of the processers is the same the systems will be windows server 2008 with 8GB RAM and 2 250GB 7200RPM Drive

Sounds like someone is on Dell's website trying to build a server. ;) For the AD server, you don't need much RAM for 50 users. 2GB is more than enough. Allocate more RAM (denser RAM as well) into the database server so at some point in the future you can add more DIMMs to accommodate whatever resource constraints are occurring. You may never need to add more memory to the database server, but the ability to do so in the future will be extremely useful.

Further analysis:

.. 2 250GB 7200RPM Drive

This is an I/O bottleneck. Using 2x SATA II drives for AD/DNS is fine. 50 users is no big deal, but for a database server it's a bigger deal.

For a database server to have any real performance for whatever may come it's way, SCSI/SAS in hardware RAID is the way to go. Yes it's more expensive. Yes, SATA II is cheaper with more storage. I realize SCSI/SAS might push the budget out of bounds, but the I/O bottleneck becomes less evident with faster buses and disks systems like SCSI/SAS. I'm just saying, if database performance could possibly become an issue, these SATA II disks are the cause. Not the CPUs. Not the RAM. SATA II disks are too damn slow.

Dual core for AD, Quad for SQL, just like the others said.

You don't need that amount of storage for AD, but having it available opens up the option of having some "dumb storage" that you can use as a software dump for your admins. I certainly wouldn't put general file sharing on a DC if at all possible, but a place to keep copies of ISO files and the like where they'll not be actively or regularly used, but will be available if you need them is always a nice thing to have.

Buy quad cores for both. If you're less than 50 people, you probably don't have a lot of servers, which means you may end up with some other function (print server) on your Domain Controllers at some point. I realize this is not a best practice, but it happens out of necessity in small business. You may also find that you can move the DC function to a lower-powered machine and use the quad core box for something (Exchange?) more CPU intensive. Bottom line - unless you're absolutely sure this will be a single-purpose domain controller throughout its life, go for the bigger server with the quad core processor.

I've decided to get 4 quadcores just because there is something to be said for having identical equipment (for rebuilding/backup server/etc.)

There is something to be said for that. However, a Quad-core is way overpowered for a 50-user DC. You could probably get away with a 4-year-old single core for that machine: the load is just not that much.

So in this case I think you'll really be better off significantly lowering the specs for your domain controller and instead spending the money on your database: perhaps by upgrading it to use 10K RPM disks or adding more ram.

Take care of these memory limit (win os memory limit)