"Purpose:
Gaming
Server
Benchmark
Daily Use
24/7"
CrysisCore - i would not recomend trying to combine all of these features into a single machine. It doesn't make any sense.
By server i am assuming a general usage machine with file, print, http and DB. in which case a p4 with 1GB RAM and 4 or 6 disks in a suitable RAID config (5 or 10 maybe depending in the importance of the data) would be able to serve 25 clients easily but unix or varient would be a much superior platform.
this specification doesn't match that of a gaming machine at all for which a single disk, 4GB RAM, windows and a good graphics card seem essential.
by my reckoning you should spend $750 on HDDs, controller and PSU for an old PC (and a new case if you need more HDD space that the current one supports) and use this for the server and 24/7 machine. spend $1250 on a brilliant new gaming rig (treat yourself you deserve it) and go on holiday with the rest
and also nosboost -- you dont need to use the same model HDDs in a RAID, they dont even have to be the same capacity. however it is good practise to make the drive specifications match as much as is possible (especially capacity as most controllers are not capable of reclaiming the "lost" capacity off the end of the RAID) eg 3 74GB disks in and a 147GB disk will look like 4 74GB disks