I think you underestimate how weak the old X2 cpus are especially when paired with a fast video card. an A64 X2 at 3.0 is about equal to my E8500 running 2.0-2.2 and will noticeably hold back a card like 4870 at 1280. a 4870 would have almost 50% of its performance go right down the drain in some games with a 6000 X2 at that res. even a 4850 will not come close to getting fully utilized in most most modern games at 1280 with that cpu so that would be the very highest card I would suggest. anything more than a 4850 at that res will likely have zero benefit in most games while using that cpu.
My overclocked 8800gt is the bottleneck with my 2mb cache e5400 cpu clocked @ 1.6 in Far Cry 2 @1280x1024 res high settings. I don't see how a e8500 @ 2.2 (which is = to my cpu @ 2.6) would bottleneck a 4870 50% @ 1280x1024?
My 85$ 9800gtx+1gb is comming in the mail this week. the 9800gtx+ is about = to
a 4850. I have Crysis,Crysis Warhead,Left for Dead,Grid,NFS shift, all the Call of Duties (Modern Warfare 1,2,WAW),Medal of Honor Air Borne,Fear. I'm sure I can find the bottleneck average.
I'll clock my cpu @ stock 2.7 (about = to a Athlon 64 @ 3.0) ,and see what happends.
I can test at 1024x768,1280x1024 and 1600x1200. My cpu will clock from 1.6 to 3.6 also.
If my e5400 cpu @ 2.7 bottlenecks a 9800gtx+ 1gb @ 1280x1024 all high settings with these games I'll be suprized.
My guess is a athlon 64 @ 3.0 will be fine for a 4870 at 1280x1024 high settings with these games. It also gives you a little insurance for future games.
On a side note do you think my cpu @ 3.6 will be a large bottleneck for a 5850 @ 1600x1200?
Edit: Take a look at the 4850 in this article. You dont need that much cpu to max it out.
http://www.tomshardware.com/reviews/build-balanced-platform,2469.html