I might try a hard reset on my router as well - see what it does. I switched it to G-only mode (instead of mixed mode) and that seemed to improve it a bit. I also downloaded the WPA2 (AES) patch for Windows XP and am connecting via WPA2 encryption. This seems quicker than WPA - really weird.
In the meantime, I'm getting the best speedtest results ever via ethernet, so I'm happy:

I reckon the lucky folks on the 40GB and 80GB will be getting their 25Mbps upgrade pretty soon - it certainly looks like they've got the backhaul capacity to cope with it.