So I just found this old Gateway computer and I wanna upgrade it and play around with it.

It has - PC800 64-MB 128Bit 400/100 RIMM
- Quantum 20-GB 4500-RPM Ultra 100 Hard Disk Drive

Now I want to add on some more ram. There is one more slot. I need to know what kind of ram I can buy and add. Does it have to be PC####?? Can it be DDR ram?? Any help?

Also. If I get a computer with DDR, DDR2, or SDRAM, would I be able to use it on that computer?

The RAM inside that computer is a 184 pin RDRAM stick. DDR, not DDR2, still uses a 184-pin system, unlike the newer 240-pin of the DDR2. However...

RDRAM and DDR SDRAM are different. Their names, most apparently are, as are their formats, are just different enough to arouse problems if SDRAM of any kind was put in your system.

You might be able to get away with some PC66-133, but that stuff is now completely dead. Another problem: Your board might only support RDRAM 800 at most, so I couldn't say. I'd get more of what is in there.

Yeah I have that same computer lol. I believe it is a Gateway Performance 1500CS. Built on 9/11/2001...creepy. You can get the memory, but it takes RIMM, which only comes in pairs usually, so it seems pretty odd that there would only be one slot available...might want to double check that.
In reference to your DDR and DDR2 questions, this computer was pre-DDR, so you would not be able to mix them effectively.

Thanks for the input, but after 3 and a half years I would say the OP is gone :D