I think ASIC is the way to go. I am just making sure. By now you can no longer profitably mine bitcoins with GPU right?
At current prices you can actually still make a pretty decent profit mining on a GPU (try the Bitcoin mining calculator to see how much you'd be making with your card). But this situation may not last long... if BTC prices fall back to where they were a few weeks ago, the GPU profitability would be very endangered. Depending on how much you pay for electricity, whether you already own the cards or not, and the possibilities of you getting an ASIC/FPGA, you may still want to go the GPU route. But know that you may lose money (as in: never have the card pay for itself).
The best long term answer is to link to a mining profitability calculator like this one from bitcoinx.
However, I think it is also important to distinguish operating costs from capital costs. At today's prices, for most miners, it is definitely profitable to operate their GPU based mining hardware. There is an electricity cost, of course, but modern GPUs certainly provide enough hashing power to justify that electricity.
The bigger question in everyone's minds right now is whether investing in new GPU hardware is worth it right now. The general conclusion is no, but that conclusion does require a few assumptions. The first assumption for GPU unprofitability is that the BTC exchange ratio doesn't continue to explode upwards. If we had a sustained $500/BTC exhange ratio, it doesn't take a lot of hash power to be profitable.
The second assumption for GPU unprofitability is that the difficulty factor is likely to increase. The graph I've linked shows the rapid rise in difficulty. If ASIC mining rigs start shipping in volume, and the BTC exchange rate stays high, then it is entirely possible that the Bitcoin difficulty could jump by an order of magnitude or more. And so investing in GPU hardware (which wouldn't even cover its operating costs at those difficulty levels) is considered unwise at this point.
So if your question is "is it worth it run my gaming machine as a Bitcoin mining rig while I'm not using it?" The answer is probably, use a profitability calculator to verify. On the other hand, if your question is "If I can't get my hands on ASIC rig right now, should I buy some GPUs specifically for mining?" the current consensus answer is no.
I still make $10 when it was at $230/btc.
Its profitable as long as it doesn't cost more than you make.
So first look at speed, if you only can make .0001btc a day (<100Mhash), you'll never make much of anything. If you can make 1btc a day you'll maybe sell it and make the exchange rate (<2-4Mhash). If you can make 10btc a day and sell it you can make the exchange rate times ten. But for all of these, if it cost more than .0001, 1, and 10 bitcoin respectively to do it. Well your loosing money.
Its not the hardware really (unless its super super super inefficient) its the speed at which you can mine.