I should also tell you that ASICs are starting to be developed and GPU mining is soon to be obsolete for bitcoin mining. The difficulty will nearly doubly which will effectively push out any low level machine that is getting under 100Mh/s

At this point it is too late in the market to seriously get into bit coin mining unless you want to spend the money on an ASIC

However, there is an alternative!

LTC, LiteCoins are another currency which can be mined by GPUs but NOT ASICs, atleast the ASICS that are about to be shipped out the door from places like Butterfly labs.

Hope this all helps.