Agreeing with above posters, i'd probably pick up a new monitor first and foremost, and secondarily a vid card. the 8800gts is really still a good card, and even a 4870 is only gonna be 40-50% faster on average.
Not sure how prices pan out in europe, but at current exchange thats about 295-445 USD. factoring about 250 for a new monitor and 200 for a new card.
If it were me, i'd pick up a 22" Samsung 2253BW at ~250, and a GTX 260 at about 200, and call it a day. But, i'm an nvidia guy. Right now the 4870 is the best card you can get price/perf so you may wanna go with that.
It's purely for visuals. If your current monitor is good enough for you, then there's no real reason to upgrade at all. Your 8800GTS-320 should be able to push most games (save Crysis) at high graphic settings with few problems at the resolution you're at.
If you're finding that you have to lower the graphic settings in the games you play in order for it to play smoothly, then it's time to upgrade just the graphics card. Personally, I have the 8800GTS-640 and a 22" monitor @ 1680x1050 and I'm waiting for the next generation of cards to come out before I upgrade.
The only reason to get a monitor is to 1. newer technology/better visuals, and 2. larger screen.
If you're happy with your current monitor, then really the only thing i would upgrade would be the vid card. A gtx 260 card can be had for about $200, or an ATI 4870, which is a bit faster, for about the same.