EN8800 GTS SLI or new card...?

I'm about to buy a new pc and still have two EN8800 GTS 640mb videocards (the one in my current pc and another that a friend gave me when he upgraded). My plan would be to combine them in SLI in the new system. This would shave off almost 1/3 to1/4 off the price of the new system. Everything else gets upgraded (from P4 3.4 to Quad core, 1g RAM to 4g,etc....)
I know these are already considered old cards by todays standards, but I'm not really looking for the best performance money can buy, just decent gaming performance! They still support DX10 and were pretty good two years ago.
I wonder whether it would be fair to say that this way I could get the performance of a recent good videocard or would you recommend to spend some more anyway on a newer card? (I'd probably upgrade in one or two years anyway, but being able to save some money now wouldn't be unwelcome...)