What resolution is he playing on?
Personally if I were getting a new card I would buy the ATI 5770, its modern, better technology, power efficient, has Direct X11 and at anything under 1920 x 1080 will run everything at max settings. It costs $149.99
I would not buy a second 8800GT, they are getting a bit old. I also would not get any of 2XX cards you suggest as they are all out dated, compatible only with Direct X 10 and somewhat overpriced.
If you are looking at getting a 470 you may as well buy the 5870 ATI card for similar price because it has better performance, is quieter, cooler and less power hungry.
I know this sounds a bit fan boyish and I apologise, as a long term Nvidia supporter it shames me to have to say that in the current market Nvidia are out of the game, hopefully they will do something about it come the next set of cards though
At resloution 1920 x 1080 it is far from pointless going with the better card, in fact I would recommend the 5850 at least.
In general, the higher the resolution the more GPU and less CPU dependant games become.
I would not go for a 5870, instead I would put the money saved towards getting a new mobo/CPU/RAM when possible, until then the 5850 should give significant improvements and when the system is upgraded the GPU can still be used. Whereas the 5770 will be at its limit on a 1920 x 1080 resolution and so has a shorter useful lifespan.
Until then, the CPU can be Overclocked for extra juice
Your best bet is probably a 5850 or a 470, but Judging by the fact that you seem to be in the UK and the prices on the nvidia cards are higher, the 5850 is probably your best bet. What are the price differences between the 5850 and the 470?
With that processor, even at that resolution, almost anything better than the current card is going to bottleneck a good deal.
Your CPU is your limiting factor. I can't imagine anything above a 5770/260 would give a noticeable return on investment. That said, if you plan on upgrading the rest of your rig anytime in the near future, a 470 or 5870 would be a great choice to carry over to a new platform.