Hey there,
For 45 pounds dont expect much. Best you're looking at is a Geforce FX5200, with a bit of searching.
Generally, the more expensive the card the better but not always. For example, the Geforce 6800 512MB is very marginally better than its sister the Geforce 6800 256MB.
I have the Geforce 7800, you're looking at 600 euros.
For a good card, I would go for a Geforce 6200 or higher.
If you really dont want to spend you'll get away with a Geforce FX series card, 5200+ retailing at less than 100 euros. It'll definately up your visuals very much from what you have now, as it will give you water + canopy reflections, anti-aliasing etc.
Plus this will probably be moved to the Hardware section, and that's where you'll find the best advice on these issues.

Good luck.