I believe you are looking for desktop DIMMs here, and I haven't owned a desktop for over six years, but to throw a pitch in here...
I have four 128Mb SODIMMs (laptop profile) PC100 SDRAM, top tier (2 Kingston, 1 Micron, 1 I forgot) available. If anyone can make good use of them you can have any or all for peanuts.
On the subject of how much RAM is necessary, it depends a lot on the OS, as well as your applications. I found that typically Windows 2000 ran great in 256Mb, Windows XP in 512Mb, and I don't use the latest Windows incantation myself but it was a dog on a 1Gb modern laptop. Those figures assume you've cleaned up the garbage that comes preloaded on a new computer and cut out many of the unnecessary autorun programs that slow down the logon process.
Like Volvohead mentions, old hard drives are slower too but how much do you want to spend upgrading? Might be better to spend $400 to get a new computer instead of $150+ on a few upgrades. If the budget is tight at the moment perhaps you can put off the entire purchase for a while.