When i am using my secondary pc for other apps ( video encoding) i run all 5x clients on my main machine
q6600 at 3.0 ghz ( could push further but shrug didnt feel like running orthos for days on end)
8gb ddr2 800 ( forget exact timings but pretty tight considering mild oc)
9800gtx
1 24" at 1920x1200
1 19" in portrait ( when im not using other pc i minimize 2 clients)
standard 7200 rpm harddrives ( 2) ( SSD is OTW)
vista ultimate 64 bit

I get fine frame rates 60 locked on main and 20-30 on alts in all areas except major citys where it can crawl tot he teens across the bored, i have all graphic setting on "high" on my main and low on alts, ir un approx 20mb of addons for each toon.
vista 64 isnt as scary as others make it out to be, yeah is a hassle disabling all the security shit but as an early adopter of xp64 it is night and day with drivers etc. xp 64 had almost zero support at launch and vista is chugging away. People talk about the resource hog at idle but dont realize thats really just vista taking ram and creating a "quick launch" of your frequently used apps. dotn fear change press on and learn new stuff.