GF4TI are the best bets for price/perf and lastability at the mo. 4200-128 are very wise but as said you may find 4400 cards cost little more, although you shouldn't consider raising the 4200 default clocks as o/c'ing as like the GF3TI200 before them they are intentionaly clocked low in order to sell the more profitable cards, most 4200-128 get from 250/444 to 285/530 with very little effort and often exceed 4400 speeds if pushed.
In any case the GF4TI cards are currently your best bet, Rad8500 like GF3 have capable perf but are hurt by the inferior perf GF4MX and Rad9000 respectively. Rad9700 is awesome, but expensive too, will devalue rapidly and by the time the speed and features truly become useful there will be faster, cheaper and better cards out anyway! The gfx card market is going at quite a pace so you may find even 2 years a little optimistic for a GF4TI, but then you should find it will sell on well in 6-12 months time and you could spend a little extra on the next best price/perf card.