Ahhh, I see. Yeah, the 5450 is a big step up from integrated graphics, although looking at those specs I still think you could run a 5550 or 5570 (they're extremely close both in performance and power draw) easily enough on that power supply, if you could still swap the 5450 for one. If not, the 5450 definitely beats the hell out of onboard graphics, and should give you better performance in videos and stuff, as well.
If you are sticking with the 5450, it might be reasonable to overclock the processor slightly. This isn't guaranteed to give you better frame rates, but at medium settings SC2 performance is still pretty reliant on the CPU and 2.6 Ghz is a little on the low side. A little poking around says 3.1Ghz is the highest most people can get from that CPU on stock voltage, but I'd probably aim for something a little lower (say 2.8Ghz or so), if possible, just to be on the safe side. AMD's stock cooler is normally pretty good, but that case doesn't have all that great of airflow to begin with and I'm not completely sure how much more heat it'll generate when you ramp up the speed. If you choose to overclock, make sure you download RealTemp, and use it to keep an eye on your temperatures.
Couldn't blame you at all if you don't want to give overclocking a shot, though, since it can be problematic, but I can conceive of that improving performance. Just to make this absolutely clear, though, if you damage the processor through excess heat (unlikely, unless you go crazy with the overclock or let things get caked up with dust) or too much voltage (and you won't adjust the voltage, or at least you shouldn't ... not with some no-name power supply) when overclocking it, it will void your warranty. Realistically, it's a small risk, and even if you do damage it they may honor the warranty anyway, but that risk is still there.
Of course, personally, when I play SC2, I play on low, even though my setup can handle ultra. Partially because it's just so much easier for me to see what's going on, but mostly because I think the multiplayer game looks better on low settings.