A 7600GS? Yeah it was time to upgrade that 3yrs ago. It depends on the games you want to play and your resolution, but I'd say a Radeon 6570 or 6670 would be a good match for that system. There's also the 5670 but it's a bit harder to find now. If it was me though, I would spend a little more on the 6770 since it falls into your budget and you can use it in your next build.
I was running a hd-4870 with a 6000+, it worked well but some games that are more cpu limited like GTAIV ran at about 25fps. It will depend on the game. Many other games ran well with that setup.
The 6770 is the replacement for the 4870, it gives you 800 shaders for a reasonable cost. The 6770 is the best bang for the buck.(or pound in your case)
No a 6000+ is from back when amd was binning their cpus really, really close. My 6000+ maxed out at 3.15ghz. The extra heat/voltage to get a measly 150mhz was not worth it and i ran it at stock clocks. 3.15ghz=140watts, its already firstname.lastname@example.org.
At stock clocks because it has 2mb cache it does pretty good.
That 6000+ cpu wont really overclock, i am talking from experience as a former owner of a 6000+.
The hd-4870 and 6770 both have 800 shaders and are similar, about the same speed.
In some games that are more cpu heavy it will be somewhat cpu limited, in those games the 6770 will only be about the same as if you had a 6750 or 6670.
In most other games that are not so cpu heavy, the games will not be cpu limited or bottlenecked. It does depend on the game.
Games like dirt2 ran at 1080P with 35fps with med-high details.
Cod duty black ops ran ok with the occasional slow down during heavy action.
Fear 2 ran the same as black ops.
GtaIV ran but it was not ideal.
Batman ran nice
Bit older games like fear, prostreet, serious samII, ect flied on that system matching the monitor refresh with v-sync on.