From my experience with the XE:
Run
memtest overnight after installing new memory.
I never got two dimms to work, tried various 1024MB ECC CL2/CL3 dimms and they only worked solo,
OS4 reports 2048MB but using over 1024MB would immediately cause glitches and crashes.
I also tried a 2048MB dimm but that wouldn't boot OS4,
1024MB is the best you're going to get I'm afraid, it's likely a bug of the Northbridge's memory controller.
Regarding USB, the motherboard USB is not able to detect USB devices being hotplugged (inserted after boot) due to missing resistors, but devices plugged in before boot do work.
Some say you can also hook up a USB hub to the motherboard ports and get hotplugging that way.
When getting a USB2 card, avoid cards based on the VIA VT6202 chipset, Doesn't work. VIA 6212 is ok and the best performing are NEC/Renesas based USB2 cards.
(I consistently got a single whole extra frame per second in Quake with NEC)
Sound Blaster Live is a good card, but there are too many variations of the "Live" card that use completely different chipsets that won't work with OS4.
CMI8738 based PCI sound cards are still plenty available as new stock on ebay and are just as good.
I also have a AGP Radeon 9250 and spare CMI8738 soundcard, if you're interested then I can send them to you for the cost of shipping.