Neither, GTS 450s look to go for about 125, so thats about $250 for a pair of them. Im not sure where you are getting a GTX 295 for that cheap, i cant find any on newegg at all right now. For $250 i would rather get something like a 6950, or if you can go up slightly higher a GTX 570 or a 6970 so you dont have to deal with SLI and some games not using it.
I would stick with the GTX 295 then, no point in spending money for only a slight upgrade when you already have a very capable DX 10 card. Now unless there are multiple DX 11 games you want to play that look significantly better in DX 11 than DX 10 i would hold onto your GTX 295 for a while and move up to a single significantly stronger card in the future.