you cant upgrade your cpu to overcome bottleneck. you might want to oc it to 3.2-3.4ghz but as long a it'ss.478 and single core and such old architecture you will face bottlenecking. the only thing you can do now is oc your cpu and add some ram and buy a new rig in the future if you dont have cash right now. anyway, 3850 is too expensive, you might buy 4650 agp. it will be a little slower but consumer less power.
I've been looking for a card for my old dell 8300 for a couple of weeks now and think I've definately decided on the HD 3850.
Is there a way to work out what spec of processor would overcome the bottlenecking problems of a specific graphics card?
p.s. Don't have money for a new rig, so looking to spend as little as possible, so no comments on upgrading to pcie please.
AceMark is right, there is no upgrade path for a S478 that would eliminate the bottleneck completely (or even by a lot). You'd need a dual-core proc to overcome most of the limits, but for your case a big OC (meaning big cooler) might be the only way.
If your PC was a socket 939, then there was some hope.