A his X1950xt 256mb
is probaby the best you can get for under $300. But i wouldn't recommend spending that cash on a high end dx9 card. Don't know if you heard(since you said you havn't been keeping up with the lastest tech) but nvidia already relased the first dx10 cards and ati (now part of amd :amazed: ) is supposed to realease its dx10 card (r600) early next year.
So i would save that cash to upgrade your system to dual core and get a dx10 card next year.
With what you already have in your sig besides your mobo, ram, cpu, i would get this:
core 2 dou e6300
g. skill ddr2 800
I don't know what you do though on your rig right now, but games next year are going to start utiliizing dual core. Im playing supreme commander beta right now and i don't think i could play it without my e6600, but my 7900gs can barely handled it on mid-settings before it starts to lag. But its still a beta with some bugs, but i still can see why dual cores are going to be a must with future games if you want high settings.
edit: if you decide to go nukem's way...i would suggest the sapphire x1950pro
instead since your running on a ati chipset. The 7900gs would still work with your mob but i doubt it will work as well as an ati card will on its own chipset.