Well...7950 or 660 Ti are not free. They are easily triple the price.
Now to the OP I agree with PontiacGTX 550 Ti SLI doesn't worth the money unless you can get the second dirt cheap.
If you want an upgrade, this is a cheap and reasonable option, you can get an EVGA GTX 560 from newegg for $130 after rebates IMO much better choice than SLI 550 Ti's
Edited by HeadlessKnight - 9/28/12 at 3:03pm