On eBay you can get 8800GTs for around £50-60 which are still rather good. 9600GT would work at 1680*1050 resolutions too buy any higher and a 8800GT is better. 9600GT go for around £50 on eBay.
If you manage to come across a 4870 for under £75 on eBay then go for it! They're worth about £90ish though. Just under the 4870 but better than the 8800GT is the 4850 which is around £60 on eBay.
For a brand new card a
9800GT is most likely going to fit your budget well at ~£69.
To be honest with any of these cards, at 1680*1050, most games will run quite smoothly with max settings and close to max AA. (Excluding Crysis obviously)