Many of us have tried multi-GPU configs in the past. I had quite a lot of experience with them up until a couple of years agi; Provided you are okay with tinkering, you can get them to work reasonably well, but I just got sick of having to tinker.
My advice is to change your attitude to upgrades; Rather than looking for a cheap second card, look for how much you can expect to sell your existing card for, and offset the price of an upgrade by that much. When the new generation of AMD cards comes out, I'd be expect a load of 7800 and 7900 cards to hit ebay.
This is my signature. There are many like it, but this one is mine. However, my signature is neither my best friend, nor is it my life. I also have no desire to master it and my worth is not signature-dependent.