Ok, so in conclusion:
get the radeon 5850 if you can spend $300, but be prepared to go a little over budget by needing an extra heavy power supply and/or upgrade to a 64 bit operating system. If you already have a good enough power supply (generally >500 watt i guess), and/or cant be bothered upgrading to a 64 bit os for a small performance gain and extra useable ram, you don't even have to get past the 300.
if $300 is really the max, get a radeon 5770 so you have some room for a new power supply and/or 64 bit operating system.

