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.