How much fps you got there?
Alpha was really screwed up for me. On i7 920, HD4890, lowest resolution and lowest settings i was getting like 20 constant fps.
Not enough fps to enjoy playing
My desktop has an I7-920 as well, but with HD5850. The alpha loved my desktop and ran 100% smooth even with everything on max, but other people with the same hardware had lags. I don't think you can judge the performance based on the alpha test.
I don't know how HD4890 compares to HD5850?