Nothing newer than X2 will install on XP. X2a requires Vista to even install the patch. The current version of SONAR does not install on Vista, Win7 and newer only.

soens
...you haven't said which Sonar you'd be running on it...

 
He said "I'd be using S5 or S6". But I'm puzzled why he would use 5 if he has 6.

He sure did, it just didn't compute... time to add memory I guess. I don't see why either one wouldn't work as the OP would probably not see any real difference in performance either way. BTW if the system is 32 bit you're limited to (+/-) 3.5 GB of ram. Not enough if you're using a lot of plugins or large VSTis - the reason I finally moved to 64 bit.
 
As for video cards, the last one I had without a fan always got too hot. I'd go with onboard if possible.

Thanks for the replies so far guys, much appreciated.
 

i cannot upgrade to current software on my XP system because of lack of support.
 
food for thought, eh?
 

I just checked inside S5 and it does indeed have the Audio - Options - Advanced check box "Enable Multiprocessing Engine".  Which is currently grayed out altogether, I assume because there is presently just a P4 installed.  So yeah, it appears as far back as S5 there is support for multi-core CPU's.  However, how well the cores are utilized, or a Quad vs. Duo will be utilized, remains unclear.     It may take a few months but once I get a new DAW configured I will report back the answer....
 
Sonic