If you pull the 512 meg sticks and put in the 1 gig sticks, it ought to auto-detect and run at the best speed it's rated for. In BIOS, there should be a memory option for command rate, either 2T or 1T. 1T is faster but most AMD systems won't run at 1T when you have more than 2 sticks of memory installed. "auto" ought to work fine, but if you find that it's running 2T command rate, you might consider manually setting 1T and seeing if it works ok. It may autodetect to 1T by itself... my system automatically chose 1T for me.
Crucial memory is pretty good, and it's what I usually buy. The last time I upgraded, I got a good deal on some g-skil memory that runs at CAS 2 (faster than CAS 3 like you bought) but we're only talking a percent or two difference speed in some applications, not enough to worry about when running AH. Crucial has a great RMA policy if you get defective memory, and I think that's more important than squeezing the last percent of speed out of the memory. If your memory dies, you just email Crucial or your retailer and they should send you replacements. If my G-Skil memory dies, I'm probably out of luck trying to get it replaced.
The second cpu core and extra memory ought to help the stutters. About the only other thing that will help stutters is getting a super uber video card with 512 meg memory which could reduce texture loads, but even then it might not help since several issues could cause AH stuttering.
After the cpu and memory upgrade, you're going to have a fine system. You might get a little speedup with a better video card or might be able to turn on better antialiasing with a higher resolution, but your system is going to be in one of those "sweet spots" in terms of overall performance. To get much more performance running AH, you would have to start spending a stupid amount of money.
