Would a GForce GT 430 be ok?

^^^^ I agree. But if you need a graphic card now then the choice is Nvidia or ATi. The GeForce GT430 works well for me in my i5 machine and is not too expensive, but even if it's not being used it drinks another 20 watts of juice, important if the machine is left on 24/7. It supports both Physx and CUDA.

I've used it on GPU crunching under BOINC (not WCG) and it works very well.

So the choice is: wait or gamble. ??????ATI or NVidia?

There is a good thread here about announcements on GPU which may happen in January, so maybe you might wait for that, unless you need to take out the bad bikers in Road Rash, over Christmas.

..or for around GBP100 ATI HD 6770 Single precision, runs at 1360GFlops!

Yeah it's better with a fan although it runs at 45-50º celsius while gaming(cod5)

VERY basically the more of these the better: "96 CUDA processing cores"

There are MANY other things that go into how fast, or how much, processing your gpu can do, but the more processing cores the better. The AMD 6770 has "320(x4)" for a total of 1280, they call them "stream processors" on ATI/AMD cards. Of course the more cores the more expensive the card. ALSO Boinc Projects are usually 'optimized' for either AMD or Nvidia gpu's, buying the wrong one now can be a huge disappointment later on. As Lawrence said January is less than a month away, see what they recommend and then buy one. As a personal experience I can tell you that over at PrimeGrid they are optimized for Nvidia cards, My Nvidia card over there is getting 30k per day RAC but when I put a MUCH more powerful AMD card over there it struggled to get that much credit, it was a 5770 which is what they now call the 6770. The AMD card worked but not nearly as well as the Nvidia card! You have waited this long give it a little while longer to see what is best, you don't want to be disappointed because you jumped the gun.