If you are having Windows XP SP2 working good and wondering to upgrade to SP3, then
I would also suggest SP3 as it has a lot of patches and updates. Your system is not bad at all, I have had Windows XP SP3 on pretty same specs with no such trouble that I feel to mention it here. Besides that SP2 as well requires being upgraded with extra security patches and updates so it's good to have SP3 already bundled with most of those updates you need on SP2.
So If you go for SP3, then I would recommend the 1st way below as it's reliable to revert back to SP2 if ever you want to get back.
1- You could choose separately to download SP3 from Microsoft's website and install it on your Windows XP SP2.
Upgrading by this way, you can have an option to uninstall SP3 if you don't like work with it. So I would say that you can try upgrading this way as if you don't like it or you feel any trouble with it you can easily remove it and can revert back to SP2.
2- If you are willing to re-install windows with Windows XP CD slipstreamed/pre-bundled with SP3.
Then you will not find a simpler way of removing SP3 and it can not be reverted back to SP2. but there are ways that you can move it to SP2(not actually removing SP3) by some ways/tricks. But i don't recommend it as it leaves a lot of crap on your system which actually doesn't disturb your computer usage but reserves a lot of disk space with no use.