RAM is not the issue, I have 2GB of RAM and I run whatever the f*** I feel like running with no lag, including Minecraft. In my opinion, it could be the processor. No offense, but AMD just isn't the same as Intel. Also, it's only running at 1.79 GHz, most processors now (including my Core 2 Duo) run at least at 2.13 GHz. Try to run disk defrags and cleanups, you probably think I'm an idiot, but it frees up crap on your computer. Just try it and see if there is a difference.
i still dont know how you run so well with little ram.
here's how it works, everything uses a set amount of data to run
processors allow that amount of data to lessen, but it's still going to use an amount of data no matter what
most of the normal functions of the computer take up at least .25 to .5 GB of RAM just so the computer can run.
Minecraft alone runs up to 1.5 gb of RAM to play so even with a fast processor you could be hitting the limit of your computers functionality.
tl;dr - processor contributes to performance but Ram is the main factor in quality