is he going with an opensource Linux OS or bootlegging WinXP Pro? ... or transferring his existing license?
There's a lot to this and you're not going to get $3000 worth of performance out of a $1000 PC, but the difference will be hardly noticeable (without benchmarking programs) if done right. 50 frames/sec @ 1280x1024 looks the same as 120 fps. With the right proc' and vid combo, you'll have your gaming performance -- they work together so don't skimp too much on one to beef up the other beyond reason. RAM may improve some aspects of loads but you want a lot of it so big games don't have to cache the hard drive often if ever, and a fast hard drive will lower load times. The moboard (ASUS is my personal preference) will improve stability of the overall system and can allow for newbie easy low-level overclocking, and the power supply ties it all together -- without absolutely stable voltage rails and lots of amperage you can experience everything from artifacts in intense games to complete shutdowns for no apparent reason not to mention increasing the likelihood of component failures (hdd's, etc). Optical drive is your choice. Plextor is arguably the best you can get, but very close speeds at a third the cost can be seen with the very reliable NEC units -- Sony isn't bad either.
Oh, and you don't really need a RAID0 -- that can double speeds in certain functions, but as already mentioned also doubles your potential for total data failure. If you go with a very fast single drive you can save a bit of money and hardly notice a performance difference at all, like the Hitachi Deskstar T7k250 250GB 8MB cache and Western Digital's newest SE 16 400GB with 16 MB cache. Both are 7200 RPM with almost unheard of access times and transfer rates (SCSI level even). RAID1 will cut usable space in half but does provide some data integrity if a drive fails, but it won't do anything for you if a glitch freaks out the RAID controller and you lose all the data even without a disk failure (happened plenty of times in my business). RAID1 also slows down write times in order for the parity information to be calculated and written, but read times improve a bit (over single drive, in most cases).