Buy a second 9600GT v. replacement card

What is your usage of such video cards? What's your monitor's resolution?
If you're a light gamer you can upgrade to HD 7770, if you wanna play games at high resolutions and high quality, you have a nice processor..you can get something like HD 7850.

If your PSU can power it, 6770 is good card, if not 7770 is way to go.....
(u will got beter dx11 support, and other goodies than SLI)

U may wanna keep 9600GT as dedicated physX ... (again if PSU can power it)

That how i would go..

My resolution is 1600X1200 on a 19 inch monitor, but I want to upgrade that at some point in the next year or so to a 24 inch monitor and a decent resolution. I mostly want to play Skyrim and Diablo III on at least medium settings if that helps.

U may wanna check D3 review in tom's looks like it not much demanding (infact there a anomaly where 6770 beat 6850, "hell yess...."), but skyrim err.... dunno...  

in 1600x1200 6770 will strugle in high detail, if u will upgrade to 24" high res monitor u may consider more powerfull card than 7770.... (means increase budged)