XP computer- worth upgrading RAM?

I have a Dell with XP installed to play older games that Windows 7 has trouble with- I have 64-bit W7 which can't handle 16-bit stuff, and XP had better compatibility with old games anyway. Problem is, this PC has 2 GB of RAM and I think that's already caused problems with one game, which locked up and caused the PC to reboot(the gog.com version of Postal 2- surprised me as I used to run Postal 2 on an old Compaq that had worse specs but maybe the gog version has higher requirements?). I may also want to install MapleStory on it as I also want to use it as a backup PC if this one fails on me.

The Dell has 4 RAM slots, only 2 are in use each with a 1 GB RAM stick. I'm getting conflicting reports on what RAM limit XP has- some say 3 GB, some say 3.25, some say 3.5 and some say 4. I will assume that if I put 2 GB more RAM in, it won't all be used? Is it best to put in a 1 GB and a 512 stick?

I'm not looking to turn this Dell into a modern-equivalent PC as I have a Gateway which exceeds it already, but I would like to squeeze as much out of the Dell as I can.

Do not mix RAM modules of different sizes, such as 1 GB and 512 MB. In theory any combination of compatible RAM modules should work together but things don't always work that way in the real world. Even modules of the same specs from different manufacturers can cause problems. For best results it is best to always use matched modules.

It's a Dell Optiplex 745 Desktop model.

I can get the exact RAM sticks on eBay that are in this PC. I have the specs and brand written down, the ones in it are DDR2, I think the brand is NANYA and it looks like the model number/type/whatever is PC2-5300U followed by some other numbers(PC2-5300U-555-13-D1.667). There's another number, NT1G64U88D0BY-3C.

Not sure if I have to match all the numbers up exact or not since some of the sticks I see are the right type but the numbers aren't an exact match. If I do need a perfect match though it look like there's a pair of 1 GB sticks for $18 shipped available.

Which would be a better idea- buying 2 used RAM sticks that match my current ones, or buying 4 new 1 GB sticks/2 new 2 GB sticks?

Motherboard specs for RAM support- Dell MM599/0MM599 OptiPlex 745 Desktop Motherboard