I would look into a CrossFire-X motherboard. They have a mediocre card on-board, but if you link a supporting, gaming-class card, the single card can use the onboard VRAM and processing power.
How hard is that to do? I should probably stress that my knowledge level of hardware is fairly low. That certainly sounds like a cheaper option though.
From what I understand it CAN be a pretty significant jump, but there can be problems (no triple buffer v-sync support unless you go into a 3 card SLI/CrossFire arrangement, some games may not make the best use of it at the moment like The Witcher 2), and in some cases you'd be missing out on more advanced features. Take, say, a DX10 card from a few years back like the 4870. If you could track down another 4870 and crossfire you WOULD gain performance roughly on par with modern cards, but you wouldn't have features like DirectX 11, not to mention you'd be using up a lot more power and generating heat than if you just got one good modern card.
Incidentally, I also have trouble finding 4870s on sale for new, which is the other problem; by the time you might want it most the extra crad may simply be gone and you'd have to resort to a weaker one in the case of crossfire (4850 for this specific example), which will limit the performance gain.
Would triple buffer v-sync be something I would have access to with a single, powerful card? From what I gather, it sounds like more games are taking advantage of it. I guess a good question to look into is, are the games I want to play able to take advantage of it?