We're trying to setup an ASP.NET Web Farm (for load balancing purposes). My question is since we've already have a Windows Server 2003 Standard 64bit, can I get a similar server but with Windows Server 2008 Web Edition.
I just want to know for sure so I can order the correct license on the server we'll be ordering. I don't want to order it with 2008 and run into a bunch of, "oh yeah, it's changed from 2003 to 2008" kind of thing.
IIS is completely different between Server 2003 & Server 2008. It might work, but I wouldn't mix operating systems in a web farm.
I would avoid 2008 if you are already running 2003. I have not attempted to put them in a farm together, but I think you would have trouble. As David mentioned, a lot changed between IIS 6 (Windows 2003) and IIS 7 (Windows 2008). There were even changes within the MetaBase, which could quickly keep things from working, unless MetaBase compatability helped.
http://learn.iis.net/page.aspx/125/metabase-compatibility-with-iis-7/
There were also many changes with how Identity / authentication is handled... I just doubt that it would even be worth the risk of wasting money to try and get working. You'd be better off just using 2003 or moving everything to 2008.
If you're concerned why not fire up some instances of each in Amazon EC2 to test before you commit? It's great for playing with things temporarily.
I tested out some configs on different server/IIS versions 2 weeks ago on it and my bill was less than a dollar which is pretty cool given what I ran through it.