Reddit reviews Sapphire Radeon HD 6570 1 GB DDR3 HDMI/DVI-D/VGA PCI-Express Graphics Card 100323L

This or this would do much better.

It seems the Core 2 Duo 4300 is an LGA 775 socket type. You could buy this processor for $78 and it should improve performance quite a bit. I have no idea how old your computer is or what type of motherboard you have, so I am not certain if your machine can handle this processor/is compatible. The socket type fits though, although this isn't the only factor for compatibility.

As another user said below, this GPU would handle Tribes, as seen in this video. (looks laggy because he's recording). Anyway, check that everything is compatible first, and if it is, this should be a cheap fix to your problem. All in all, it should cost you about $150-160.

CHECK YOUR COMPATIBILITY FIRST.

Edit: If you need any advice, join us over at /r/buildapc.