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.