I'm hardly an early adopter, joining up 9 whole days ago. What made me sarcastic and condescending (not to mention bitter) was answering this exact same question about 30 times.
If we assume the difficulty will grow by 30% every 2 weeks (both of these estimates are very conservative, by the way), 700 Mhash/sec worth of hashing power will probably find about 1.95 blocks between now and the end of time.
You'll have to plug in what you think a bitcoin will be worth to find out if this is a good idea or not.
You'll need to scale that to the GPU you are planning to buy. If, for example, you get a 5850, you can expect about 300 Mhash/sec out of it, so it would earn about 1.95 * (300/700) = 0.835 * 50 = 41 BTC. Even that estimate is probably very high.
See that is what I was looking for...Some data on what Mhash/s I could get out of various cards...And I couldn't find any place,with a list of cards and their respective perfomance...