I previously had a decent Iiyama 22" panel, which I decided to hand down to my son. I now think that this is putting too much strain on his system (another hand-me-down

He is running an Athlon 1800XP with an ATI X800 with 512MB of old school DDR. It was previously running on another Iiyama - a good old 19" CRT. It has always been sluggish, but now it's more of a fossil than a slug. Here's how it goes.
On startup, the PC sounds as if it's behaving normally, although there is a good minute or two before the single "everything is ok" POST beep. Then, nothing is displayed on the monitor until the "Welcome" screen (XP). No part of the POST or BIOS info screens is displayed at all.
When the desktop eventually loads, everything is extremely sluggish. Open Firefox, for example, and nothing happens for a good 3-4 minutes. A similar wait occurs using Ctrl-Alt-Del to check running processes.
Is it just me trying to run a native 1680x1050 monitor on a low-end system? Will I have to downgrade to the good ol' 1024x768 CRT?
Or, hopefully, is there anything I can do to get this configuration running with any amount of stability?
(As an aside, once I get some life out of it, I'm gonna run CC on it - the amount of crap he's downloaded is staggering! Guild Wars addict at ten years of age ><)