Moderately ancient computer help

[Batman]

Ace: Yes I know my rig isn't up to date. Send me a ton of money or shut up.

So I'm currently using an AMD Athlon XP 1700+ on a Asus A7V266-E mobo and as it looks like I'll have a little money coming my way I thought maybe I'd upgrade to the extent that is still possible with a system that old (was built in late '02).
Given that the latest BIOS update says the board supports up to a 2600+ CPU, I started looking for those-except they apparently use a 333 MHz FSB. Which from what I understand, the board doesn't. I found several articles about 266 MHz 2600+ CPUs, but not a single place where I could BUY them.
What I DID find were 2800+ CPUs with a 266 MHz FSB. So what are the chances of those running in my system?

Second, I currently have a (by now) measly 512 MB of RAM. I CAN still get 266 MHz DDR DIMMs for reasonable money, but only in 256/512 MB. My system supports up to 3 GB as is, possibly more with the latest BIOS (ASUS download descriptions leave something to be desired, but with HW that old I suspect so does everybody else's). I HAVE, to my not inconsiderable surprise, recently succesfully installed a 400 MHz 512 MB DIMM in a 266MHz system (ALONGSIDE a 266 MHz 256 MB DIMM) and it appears to be working 5-by so far. So did I just get incredibly lucky, or will faster RAM simply work but spend a lot of time lounging around in a lower memory speed system? I'm afraid I'm a little behind the times these days.