get a 5870 I could make a good 730 MHS and would be a bit more worth it at around 130 a month.
not sure where you are getting your numbers.
a 5870 will range from 340 mhash/s to up over 400 depending on a few factors: http://en.bitcoin.it/wiki/Mining_hardware_comparison#AMD_.28ATI.29
so even if you got 440 mhash/s from it, let's say, that's 7.45 BTC/month at current difficulty, or about $82.00 usd -- and that is assuming difficulty does not increase: http://www.alloscomp.com/bitcoin/calculator.php
i don't mean to say it won't be a good move, just wanted to make sure this was clarified.
also, your title makes it sound like the only value bitcoins have are for mining and selling.