Our project are plan to migrate from Sparc to x86, and our HA requirement is 99.99%, previous on Sparc, we assume the hardware stability would like, hardware failure every 4 month or even one year, and also we have test data for our application, then we have requirement for each unplanned recovery (fail over) to achieve 99.99% (52.6 minutes unplanned downtime per year).
But since we are going to use Intel x86, it seems the hardware stability is not so good as Sparc, but we don't have the detail data.
So compare with Sparc, how about the stability of the Intel x86, should we assume we have more unplanned downtime? If so, how many, double?
Where I can find some more detail of this two type of hardware?
Intel's Xeon 75xx series chips inherited 90%+ of the RAS features of their thoroughly enterprise class Itanium chips when launched last year. Their stability, especially when coupled with 75xx-aware OSs such as Server 2008 R2 and RHEL 5.5, is significantly better than their other x86 counterparts. These chips are available in servers by all the main vendors such as HP's DL580 G7, DL 980 G7 and Dell/IBM equivalents. Hope this helps.
One of the advantages of x86' relatively low cost is the ability to scale horizontally. Your mission-critical applications should be able to survive a complete failure of any component, including one system in a cluster of systems.
Since the x86 platform is a lot bigger, it's only natural that the spread between top-notch and bottom-feeders is also bigger. In the Sparc architecture, you'd be hard-pressed to find cheap low-quality machinery, in x86 you can find plenty of it. This is both between brands/manufacturers and in models within a single manufacturer's range.
However, there is also plenty of quality x86 kit arround. All of the reputable brand vendors have mid to high-end systems that are quite capable of reaching 99.99% (after a burn-in, not counting non-impacting defects, and you didn't specify how many SPOF's you intend to chain). Any good account manager will put you in touch with (sales-)engineers to flesh out your actual requirements.
Since you appear to be doing a major upgrade, it would be worth it to investigate reliability via software. There is a whole range of companies doing cheap ("somewhat" reliable x86) hardware with smart software to achieve Very High Availability. Some of their work may prove to be easy yet valuable for you.