Here's what I was able to accomplish from the stock speeds:
GPU 875 MHz
Memory Clock 1335 MHz
Not sure how great that is. I scored a 12590 in 3DMark06 with it whereas stock speeds gave me 11328. And that was with just using the Catalyst program to test and find the supposed "optimum" settings.
I eventually want to upgrade my card, but seeing as I just plonked down around $250 for a new motherboard, CPU and memory, I'll just have to suffer with my HD3870 (which actually isn't doing bad at all).
UPDATE: Looks like I'll be overclocking my 5750 next. Just wish it would get here soon.
Well bummer. The card started showing artifacts. I'll have to keep working with it and see when those go way. They don't show up in most games, but they definitely show up in GTA IV as a bunch of red + signs.