Third Way, you may have seen this already, but if not, then definitely check it out: https://en.bitcoin.it/wiki/Mining_hardware_comparison
The main column of interest is the Mhash/s column. The higher the number, the faster you'll mine bitcoins. ("Mhash/s" is that number of millions of hashes per second that can be tested.)
The other important number can be found here: http://blockexplorer.com/q/hashestowin
That is the average number of hashes that you'll have to try before successfully mining some bitcoins. Note that the number isn't constant (it goes up every so often as more people start mining).
To calculate how many bitcoins you'll mine, take the hashestowin number and divide by the number of hashes you'll try per second. That'll give you the number of seconds it'll take you on average to get 50 bitcoins. For example, if you have a top-of-the-line computer with a single ATI 6990, you'll be doing somewhere around 750,000,000 hashes per second (per the wiki link). The current hashestowin is 1048471149936089. So divide 1048471149936089 / 750000000 ~= 1397962.
Then convert 1397962 seconds to days: 1397962 / 60 / 60 / 24 ~= 16 days. Each time you win, you get 50 bitcoins. So you could expect 50 bitcoins per 16 days or so.