As usual my question is AMD/ATi or nVidia?
But, differently from many, I'm not interested in particular models, first let me introduce my case.

I'm currently having issues with my notebook (it costed 2000 EUR in 2007 ) because when the CPU (2 cores) stays at max 2.4 GHz for more than 2 minutes (even 1 only core) Linux decides it's time to shutdown, so it gracefully performs a mandatory quick shutdown.
The other day I was playing the beta of HoN and this happened during a game; I've used this PC to play WoW (with wine) for 1,5 years, and as it is now, I could definitely not play it.
I don't use windows, if not in virtualized mode only when I'm working from home (my employer now supports RDC with Linux now as well...).
Maybe I'll buy HoN, and probably (if I'll get a new pc) SC2.
Clearly I will use wine to play these games, I'll refuse to boot in windows for whatever reason; at bad I'll play games on my PS3.
Btw, now to play HoN I have to set processor speed to 1.6 or 2 GHz, otherwise OS will shut down.

Usually I have a dual screen:
1920x1200 + 1680x1050

as now, my notebook is a nVidia 8600 GT Mobile with 512 VRam and I have to be honest with you: in 2+ years it always worked 100% and TwinView is a magic software: it works.

Other uses I do of my videocard are to do experimental GPGPU: I've used old school OpenGL till now, but I'd like to actively switch to OpenCL asap.

So my case is:
Gamer+OpenCL dev+Dual screen, Linux (Ubuntu) x86-64 only (from a moral standpoint I do refuse to buy anything from MSFT).

My target system will be or a i7 quad core, or a Phenom II 4 core with 8 of 16 GB ram, 2 disks (~0.5 TB and ~1.5 TB).
I'm still undecided if pay a premium for i7 or save ~200 GBP and have a processor which will consume less power; anyway the target figure is around 1100~1300 GBP.
I'd like to spend 250~325 for a videocard. What do you recommend?

Please elaborate!

Thanks again,