"....I checked again and SIW definitely says that the speed is 66MHz."I have no idea whether you're looking at the spec for the right thing, or whether SIW is reporting it wrong.
I can't think of anything on your mboard that would use 66mhz except the AGP slot.
If you remove the AC power to the case, remove the ram, if the ram is a major brand you can probably nail down it's specs by using it's module ID string to search with on the manufacturer's web site, or on the web. If it's generic that may not be possible and we'll need more info from you.
When you place the ram in the slots, make sure it faces the right way so the notch in the bottom of the module lines up with the bump in the bottom of the slot, and that both latches at the ends of each slot are against the module. Plugging one in backwards will fry the slot and the module instantly when you attempt to boot the computer and can neither can ever work again.
"Will it make any noticable difference if I add 512 MB? "
The 512mb you have is already a decent amount.
Whether 512mb more will be noticable depends on what programs you have and what you do with your computer.
Ultimate Memory Guide
How Much Memory Do You Need? etc.
http://www.kingston.com/tools/umg/u...
"...can I just add one or the other to what I already have?"
Yes, but keep in mind all the ram will run at the specs of the slowest module.
For the same total amount of ram, you will be better off to remove the slowest one and use the larger 1gb module if you get only one, but on the other hand if you have a situation where you actually need more ram, more totqal ram running slower is better than less total ram running faster.
Your mboard supports installing matched pairs of ram so they can run in dual channel mode, but the dufference is tuiny and not noticable most of the time, so you don't need to fret about that - more ram, if you have things that benefit from that, has a lot more of an effect than whether it runs in dual channel mode or not
This mboard supports up to 4 1 gb modules but the 32 bit Windows version most people have cannot deal with more than 3 to 3.5mb of ram, so there's no point in installing more than 3gb total for 32bit OSs.
......
A tip.
Your onboard graphics are decent, but your ram has to share ram with it, and that greatly reduces the ram's max bandwidth. The bandwidth is directly related to the ram's max data transfer rate.
Using even an inexpensive graphics card instead allows the ram's bandwith to reach the max banwidth it was designed to use, e.g. in tests I did in Sysoft Sandra that was double that of when the mboard was using onboard video.
That will make a noticable difference especially when the computer is taxed more even with the same amount of ram.
