Also, if you remember your make/model, go to the manufacturer's website, punch in the model info in the support section, and see what they gave you, it could possibly be under 'technical documents' or similar. Or google your make model specs, eg Dell Dimension XPS 5400 specs... just make sure you're looking at factory specs, not someone's customized one.
Already a step ahead of you

I just went to crucial.com and put in my computer -- compaq presario 1625nx, apparently it comes with 256 standard. I distinctly remember there being 2 modules inside, however, because I unlatched one of them yesterday while installing the video card.
The second is definitely something I added myself, most likely from my past computer. If I had to guess, I would say that right now my memory is probably not running in dual channel. I think one of the modules might be PC2100 and one is PC3200.
I could easily go get a 1GB stick or two 512GB sticks. However, if I get the two 512 sticks, I'll be up to 1.5 GB and possibly 2 of the 4 sticks would be running dual channel. On the other hand, if I get the single 1GB stick, I may not be able to boot up with 3 sticks in there and I'd have to get rid of one of the 256's, which would leave me at 1.25GB, also not running in dual channel because it would not be matching sized modules.
I'd like to minimize travel time, if possible, and avoid driving around Philadelphia at rush hour when school is right next to the store. I am guesing at this point, the 2 matched 512 sticks is probably the best/safest bet? Best case scenario, the 2 existing sticks are already in dual channel and the new ones are as well, and I have 1.5 gb at dual channel. Worst case scenario, I am at 1.5 GB without being at dual channel.
Question about that, however -- would it be a noticeable upgrade to add 1gb of ram if I do not receive the benefit of dual channel?
At least I can rule out buying a single 512MB module, because in that case, I can't boot up with 3 modules, and I'd have to lose one of the 256's and I'd definitely not be in dual channel.
...and I suppose while I'm here, may as well add another option -- getting 2 additional 256mb modules?
Hm, the more I write the more I realize I should go home and see if the existing ones are in dual channel or not. That will probably make it a lot easier. Final question (for this post) -- would it be preferable to run 1GB in full dual channel or 1.5 GB not in dual channel? (buying a matched GB of 512's, or even just a solo 1GB module and losing the 256's altogether might be a final option).
Haha...what a can of worms I've opened up.

7600GT should be more than able to handle it, I guess, but you could try without the -env flag -- environment mapping tends to be one of the heavier options.
It's definitely something I could experiment with in a couple of hours.