I need some advice for a new video card for my pc?

Dimension XPS 400, Intel Pentium D 820 (2.8GHz) w/Dual Core Technology

1GB DDR2 SDRAM at 533MHz (Upgraded to 3GB’s)
256MB PCI Express x16 (DVI/VGA/TV-out) nVidia GeForce 6800
250GB Serial ATA Hard Drive (7200RPM)
Microsoft Windows XP Media Center Edition 2005

And this is what i want my pc to be able to do:
I’d like to be able to use my computer as a dvr, and while i can do that now, i’d like a videocare with an hdmi port as i have a hdtv. I’d also like it to have a dual tuner so that i can record 2 shows at once (if possile). Any suggestions or advice would be greatly appreciated. My budget is around 300.