CS:S used to run on max settings on my P4 with a 64MB VGA. Ah the good old days.
The GPU is good enough, and the CPU should hold up but a bit underpowered for the HD6870 IMO. The best AMD deal these days is the 955BE.
Check the $800 build on my sig (which I believe is about £500?). The i5 2400 will be much stronger at Total War.
Cuecue its 1366x738 i think and zero on amazon (uk) the prices are the same but in a diffrent currency if you get me.. so that build would be £800 in pounds
It will have no problem with that resolution, actually quite overkill. I think building am3 is not a very good idea at the moment. I would either go Sandy Bridge or go with an am3+ build to be ready for the next generation of AMD processors.