.....& that I don't know..
Mismatched memory sticks might work (unlikely), depends on the motherboard, but even if they do that's not the way to get the best performance. It's better to have matched sticks, and you also have to make sure the BIOS can handle their timings. If you don't know what that means, take the machine to Regina Computer Exchange and ask for their advice, it's the best shop in the city in my experience, I don't go anywhere else anymore. I don't buy computers, I buy components from them and build my own, and they know what they're doing.
1 Gb of RAM's a bit low for XP, and adding RAM to the configuration you have is almost certainly the best bang for the buck to improve performance. XP won't see 4 Gb though, it'll probably tell you it sees 3.5 to 3.7 Gb, owing to some peculiarities in the way XP addresses memory. Second best thing to do, and by far the cheapest ($0) is to kill off some of the extraneous processes. Over 50 processes is probably about 20 more than necessary. That's an HP system, and they come bundled with all kinds of stuff from HP that isn't necessary. Check out this guy:
Black Viper's Operating System Guides. He's probably forgotten more than I'll ever know, and his advice is sound. On his site you can find lists of all the services XP runs, what they do, and what you can safely stop. Anything you see in your process list that he doesn't mention is something proprietary to HP, or part of some of the apps you run. Some of them may be necessary, like if you have an HP Lightscribe DVD drive there'll be a driver for it (but it'll be necessary only if you use the Lightscribe feature), but a lot of it is likely to be stuff you don't need to have running. HP has a "PC Health Check" app, for instance, and an updater for all the HP software on the machine, and online gaming processes, and apps for managing photos and videos, and so on. If you don't use them, you don't need those processes to be running.
And seriously, if you want some on site assistance, PM me, I'd be happy to come to your house and talk with you about your machine. After the advice you gave me about furnace service last winter, I owe you one.