shadow or base memory in bios

[harvyne]

i am running windows xp and in my bios there is an option for usb memory to be set at shadow or base(640k). can anyone tell me the proper setting for this and what purpose this serves.

gigabyte ga-k8n pro sli
seagate 320gb 7200rpm, maxtor 120gb 7200rpm hdd's
amd64 4000+ cpu
2 gb ddr supertalent pc3200 ram
bfg geforce 8800gt video
soundblaster audigy
liteon dvd
coolermaster 500w psu