I've done a lot of reading and calculating, and I think it's too late to get in on mining if you're planning on buying a new computer to do it, and running that computer 24/7 when you otherwise wouldn't have done so.
Personally, I already have a computer that has a good ATI graphics card (only one card though
) and that computer is already on 24/7 for other reasons. So I'm using a tiny, tiny bit more power by having the graphics card spun up all the time, but not that much. For me, bitcoin mining is just a way to make a machine I already had earn a bit of money.
Is there any way you could cut the cost of that machine you're thinking of building? If it's just for Bitcoin then you don't need anything fancy, just two bare-bones rigs with the bulk of the cost going to the GPUs?