Fisico, I have a similar spec pc to you (see sig), and i get about 30fps in 1024x768 - pretty darn good. I would suggest that you upgrade (or downgrade) the drivers for your GF4. i use 30.82 - went down from the latest ones (40.something) and it really made a difference. I think the newer drivers are designed for the Ti series cards.
On the subject of upgrades -
a)more RAM is the cheapest option, and will certainly help - another stick of 256mb will do wonders
b)more expensive, but even better!
c)yes, shouldn't have any problems