You could look for a PCI-E 2.0 video card with 1G GDDR5, 128bit, and a Bandwidth 80 - 128 Gbyte sec. From reading player's reports since the closed alpha starting august of last year and my own card. You "might" get about a median experience with the video card maxing your old system's ability to support it.
9600 GT
RAM - GDDR3
Data Paths - 256 bit
Band Width - 57.6 Bbyte sec.
9800 GTX
RAM - GDDR3
Data paths - 256bit
Band Width - 70.4 Gbute sec.
GDDR3 is about 2x slower than GDDR5. And so to your low Band Width which needs to be much higher. Did you end up installing the alpha and getting about 6-7 FPS sitting on the runway?
Are you XP 32bit and your CPU, 1 core, 2, or 4? I tested the closed alpha on my wife's single core Pentium 4, XP 32bit and a 9800 GTX 512 GDDRE3 card. Very low FPS with everything turned off or down.