Here's a benchmark Tom's Hardware did with the ATI 5670. They compare it to the 9800GT you already have.
Benchmark Results Start Here:http://www.tomshardware.com/reviews/radeon-hd-5670,2533-6.html
Yes the ATI 5670 can be overclocked, but it has HALF the Texture Units and HALF the ROPs
compared to the ATI 5770. So it will not perform the same even with the same clock speeds.
Granted the benchmark above is using January 2010 drivers, so I'm sure there's been improvement since then. But considering the ATI 5670 is basically at or just below the 9800GT, I'd say it'd be a poor choice.
As I said earlier, to get any of your money's worth, you want an ATI 5770 or GTX 460 before it's worth spending money over what you already have.