NV20 = Gf3Ti
NV25 = Gf4Ti
And now:
As you can see GF4Ti4200 (130-140$ in
http://www.pricewatch.com) can be overclocked to give stock GF4Ti4600 a pretty good fight. Just be sure if you're gonna buy GF4Ti4200 card it's having fast mem-chips like MSI I'm using (3,6ns Hynix memory). Card runs stable at 300MHz core/600MHz memory when Ti4600 usually has 300MHz core and 650MHz mem-chips.
Last days I've been playing MB-Truck Racing, Rallye Trophy and Il-2 with full details , 1600x1200 resolution and 32bit colors with AMD 1,4GHz Thunderbird and I haven't seen any framerate drops yet...
You just CAN'T go wrong with MSI GF4Ti4200 card (No; I don't have their stocks

)