I have 512 MB RAM installed will adding 1GB more make a huge difference?

You definitely need more ram as 512 Ram is simple not enough for even the smallest amount of applications even I PC right out of the store has at least 1 gig of Ram if not more and you will find the even that is to less for a PC know a days. Buy just upgrading from 512MB Ram to 1 Gig you will not see much of a difference if any at all you should have at least 2 gigs of Ram installed. I terms of what Ram to use in your case if the PC is 3 to 4 years old you should be looking at DDR 2 Ram most likely. A even better way to tell would be to simple take a stick in and ask a sales person as they would know just by looking at the Ram as there are a few marking on Ram that tells you the type of Ram that it is and go from there, and for installing it you should do yourself as it is as easy as slicing a piece of pie. And for the record just to let you know if you are using Windows XP there is no need to go over 4 gigs of Ram as Windows XP will only recognize up to 4 gigs of Ram but Vista can run more and for breaking your PC by using the wrong ram you can't as it fits or it don't fit at all and if there is a limit it would be at 2 gigs if there is one at all.