I have just gone though this and would definitely recommend the IV over the III. UDMA is the future little or no advantage in most cameras but the upcoming D3 D300 and I am sure for most future SLRs it will be 3 times as quick. Also if you get a UDMA Firewire 800reader in the real world it's 4 times as fast. 2GB a minute which will make life easier. I posted
here and taking postage and VAT into account this was the cheapest I could find for the extreme IV and the reader.