Almost all standard modernly-built computers can play pc games and dvds, though some games will require good graphic cards. Having the idea of building a “supercomputer”, it’s clear that today’s most powerful (and expensive) processor is the intel i7. I don’t know much about graphic cards, but it’s advisable that you use two of them (SLI or Xfire configurations). You should know that the graphics card is more important than the processor in order to play knew games: I mean, you should have a standard processor (not a low-budget one) and spend your hardly-earned () money in two good graphics card.
In order to play PlayStation or Xbox games, the key word is EMULATORS; and maybe you won’t get fully compatibility with games. For instance, Epsxe is a great psx emulator that even works fine in legacy (Pentium II and III) computers. However, PlayStation 2 emulators will require a good graphics card, and there are many compatibility issues with them (I mean, some games don’t behave as expected, and some don’t even work at all). I don’t know of any Ps3 emulator, but I think you’ll probably have to wait several years before a good ps3 emulator is made.