2GB of RAM for Vista 32 is typically very usable for every day light computer uses.
If you use one or more moderately demanding applications with substantial amounts of data involved, you can EASILY reach a point where you'll see substantial slow-downs due to memory exhaustion.
Examples might be:
* typically running a few dozen browser windows at once.
* using something like photoshop to edit big images with lots of layers, etc.
* using outlook with huge email repositories.
* copying / virus scanning lots of data while you're doing things like office applications in the foreground.
* keeping some big office applications open while you're running outlook, a few web browser windows, having a few PDFs/documents open, playing some MP3s, etc. The combination of a few things like that can slow you down.
...but for light / casual use and doing mostly one or two thing at a time, 2GB is going to perform OK.
On the other hand, DDR2 memory is DIRT CHEAP (i.e. almost as cheap as it ever has been or ever will likely be on average). If you HAVE one or more UNUSED slots for more memory, $40 will get you an 2GB DIMM you can add in to your existing 2, that's a pretty cheap upgrade and will be about the best bang-for-the-buck future-proofing and performance boost you can do with an existing system like that.
Personally if I think I'd like to keep the PC in use for more than a couple of years (either for myself or eventually for use by a family member or whatever), I'd say 2GB will start to be enough of a limiting factor that spending $40 or $80 or so to add another 2GB or 4GB is well worth the investment for payoffs that'll be present every day for years to come. Basically it seems worth it to me (at these fire sale RAM prices) to MAX OUT the memory in ANY PC you think will be a workhorse for you.
If you have only 2 memory slots would I take out and throw away new/good RAM JUST to upgrade to 4GB? Well probably not unless I knew I'd be using it heavily enough that 2GB would waste lots of my time and limit my work. Maybe if I had another PC of mine or a family member's that the taken-out 2GB could be used in for a much needed upgrade or something then it'd seem like a good overall deal without wastefulness.
Since you like buying things at good deals on sale, I'd submit that's exactly the situation for DDR2-800 and DDR2-667 memory right now, so even if you can't use the meory for THIS PC, buying some of the best deals on quality but cheap 1GB or 2GB capacity sticks NOW for future use personally or as gift upgrades for family members or whatever isn't a bad idea.
Any PC that's expected to have modern applications on it deserves more than 1GB of memory, and 4GB ideally for power-users, so plan accordingly for what makes sense for your computing needs, budget, and overall plans for computers in the household for the next 2-3 years.
