1. Does RAID offer real performance gains in gaming? I've heard that adding two drives together can actually make them slower in some cases...
Firstly I presume by "RAID" you mean RAID0. In answer, it depends on the game. Games with large levels can benefit from RAID0 thanks to the increased sustained throughput. As for making them slower, that's fairly rare. You can set up a raid array in such a way as to deliver poor performance, but generally that'd be user error, not down to the technology per se.
2. Is RAID easy to set-up for someone that's never built a PC before? Is there any specialist knowledge required?
You need some knowledge about how it works and what it does, as with most things in life. However, if you're patient and prepared to learn then you should be fine. If you're building PC's then you should be able to cope fine with RAID etc.
3. Would two Western Digital Raptor X's run in RAID better than two Seagate Barracuda 7200.10's?
"Better" is such a slippery term. Raptor's are better at seek intensive jobs (random access to lots of files or inside one big file), while the Seagate's are slightly better at sustained transfer rate on the fastest part of the disk, although at slowest part the Raptor's are again marginally faster, if I remember correctly. For gaming, the Seagate's are the better buy, since the seek time advantage is probably not really all that important (especially if you keep the drive/array defragmented thus avoiding/minimising seek overhead) and you get a lot more space for your money to boot. But, for some other applications (for example software development with lots of little source and object files being read and written often) the Raptor's will be better. Aside: The Raptor X has a clear window. While it looks cool, the drive costs slightly more than the normal Raptor, and also has a slightly reduced MTBF rating. I'd thus suggest staying away from them and just getting a normal Raptor if you're set on getting a couple.
4. Any other advantages / disadvantages I should be aware of?
There's the often mentioned (but IMO overemphasized) truth that using RAID0 increases (with 2 drives doubles, with 3 triples etc.) your chances of data loss, because if either of the drives break down, the data on the entire array is lost. However, the likelihood of one of the drives breaking down remain pretty small really, and in any case if you have a suitable backup regimen (as you should have even if you're using a single disk) then the increased risk is largely a moot issue. Nevertheless, that's something to be aware of.
