Okaaay - that's a little more than the average duck
A couple of things to note:
Is highly likely the RAM which came with your system is little more than bare-bones, and certainly not up to what you are doing. Being that you are here in Brissy, have a look on
Megabuy for the best compatible RAM for you sys - G.Skill and Kingston's HyperX series. Can also use shobot.com.au to shop around.
x64 Windows - never used XP x64, but recent move to Win7 x64 have really only had a couple of things which wouldn't run, the only real pain being Creative's software stack for the Zen (although they did provide driver through WinUpdate, so can at least use Zen Explorer) - but that's highly typical of Creative really

A couple of drivers had to be "signed" this end using a free utility. The emulation layer built into x64 Windows should allow just about any x86 app to run (with the exception of AV's and the like). Only limitation is those old 16-bit apps, which got dropped with Vista from memory.
Also, you have to consider a couple of limitations in Ubuntu (this is not a crack at Ubuntu by the way - despite my feeling toward that flavour of *nix) :
- Ubuntu was simply never intended for that kind of loading - it lacks the hardware optimisations/accelerations which you really need for that type of loading.
- Most open-source/reverse-engineered drivers tend not to be full-featured, again giving a performance hit. SUSE 11 might get you closer, but again with limitations.
- Running apps through WINE usually means reverting to older builds of key apps, meaning lacking any more recent performance optimisations.
On the other hand, hardware accelerations intro'd in Vista have been massively refined in Win7. Given that both nVidia and AMD
both pushing Win7 as next gaming platform, might at least be worth giving a crack? Not being a fanboy here - absolutely HATED XP, which damn-near sent me to SUSE. My concern is that if are throwing such a heft load at Ubuntu - to the point of crashing RAM - it might be worth looking elsewhere.
BTW - for those wondering why OS X not mentioned... yes I know Leopard and Snow Leopard also contain hardware optimisation, but
cwarn23 is on a PC and can hardly suggest the "hackintosh" methods here