as far as I know you don't need a 256Mo graphic card cause the card is barely used by Resolume (only using Overlay or DVDMAX function but not the gc RAM so even a 64 Mo cg will do the Job )
Also You should go For a P4 HyperThreading instead of a 64 bits AMD cause Resolume don't use 64 Bits optimisations and is all about Ghz frequency (wich is bigger on P4 than on anythin else including Pentium M or Celeron or Dothan)
Maybe You can use 3Go RAM with RamDrive but I advice you to test before, I did and the results where not good, instability, lockups and not easy to use (not complex but you constantly have to move your files, not very feasable when you're performing...)
Actually a lot of us are waiting for a better RAM managment system included in Resolume wich would allow a lot easier RAM usage (by loading the playing clips in RAM) so you don't have to move files constantly, we can only hope that feature will come soon, Edwin is working on it but we have no release date (with V2 would be great but I'm afraid we'll have to wait longer...)
Raptors 10.000 rpm is definitly a good choice, If you go for RAID I'll suggets you get a RAID-MATRIC capable chipset so you can do RAID 0 + 1 with only two Raptors (these are quite expensives...) and you get the best of two worlds : data security/protection and data transfert speed
For onboard Midi I don't think it's required have to have it onboard , nowadays there are many MIDI-to-USB converters or even MIDI hardware who have built-in MIDI-to-USB
My final advice: Don't try to build the "Bigger" (like "Mine is bigger than yours nanana...") PC
Read the Forums and see what Resolume Really need and take that as the basis of your future PC
I want to put it all in a 19" rack mountable case - it'll be on the road a lot. Must have good audio in's too for effect driving.