RAM. It's damn important.

RAM - Random Access Memory. Never underestimate its importance.

Here is my little guide to Operating Systems vs. RAM requirements for Microsoft Operating Systems. Please note that the advertised RAM requirements are 32MB for ME, 128MB for XP, and 512MB for Vista. That may run the O/S, yes, but for actual use of the PC, you're gonna need a lot more than that!

ME/2000 - Advertised as 32MB RAM minimum. I recommend 512MB minimum for acceptable performance. 768MB is plenty, and 1GB maxes out most systems with those Operating Systems. Bear in mind your system probably uses SD-RAM which ain't cheap, plus some motherboards from that era may not support more than 256MB per slot! If you've got 256MB of RAM, your PC will struggle when opening more than 3 or so applications together, so it may be worth upgrading by another 256MB at least.
You may also want to look into buying a new PC...

XP - Advertised as 64MB absolute minimum and 128MB RAM recommended minimum. Nowadays, 512MB just about cuts it. My recommendation would be 1GB minimum for acceptable performance coupled with running multiple applications. XP machines were initially shipped with DDR RAM, which can be found for roughly £25 per Gig. I've seen it as low as £17. If you bought an XP machine post-2005, it will most likely be DDR2 RAM installed, in which case, max out your system. It's dirt cheap to do it - roughly £8 per gig. Install 4GB max if you run the 32-bit version - it will come up as 3.2GB but the remaining space can be used by the system for address allocation. You can max out your system with no hesitation with the 64-bit version.

VISTA - A whole new kettle of fish. Advertised as 512MB minimum (Home Basic) and 1GB (Home Premium/Business/Ultimate). Yes, this amount of RAM will run your installation of Vista, but if you want to start running multiple applications, forget about it. My recommendation - 2GB of RAM minimum for Home Basic, and install as much as you can for the other versions. 2GB will usually run Premium quite well, but to get the most out of your system when you've got multiple applications/games going, it would be worth installing more. The good news is that you'll most definitely have DDR2 RAM in your machine, and you can cheaply install heaps of it.
If you've got 3GB (2GB + 1GB) on a 32-bit system then that would also be ample.
For 32-bit, don't install more than 4GB - it will come up as 3.2GB but the remaining space can be used by the system for address allocation, and for 64-bit, you may max out your system if you desire.


Finally, a note on Windows 7. This version has a 32-bit edition and 64-bit just like XP and Vista, though the 32-bit will be rarely used. This is great news as manufacturers now have no reason not to install 4GB (or more) of RAM. In which case, you'll not need to worry about RAM requirements as Windows 7 isn't as heavy on system resources as Vista.
You can of course install as much RAM as your system can handle with 7.
Also, expect to see DDR3 becoming the standard over the next few years. Currently, there is not enough demand, so the prices remain high. But as manufacturers start using DDR3 modules, expect to see the prices fall.

RAM is awesome.

Digiman out.