Your computer ought to play D3 at almost-high settings. Your 9600xt has the horsepower to enable all the fancy effects, obviously without AA or AF and while using lower-ish resolutions. I'd figure that your system could do D3 at 1024x768 at better than 30fps with at least medium game settings if not more...
You really only need UBER hardware if you want to play at high settings along with high resolutions AND high refresh rates. If you're ok with 1024x768 at ~30fps, then your rig should be just fine.