UDMA (ultra direct memory access)
is the definition of a new protocol for the interface between the hard drive and the system.
It improves upon the ATAPI/EIDE standard by doubling data transfer rates to 33MB/sec, enusuring faster disk reads and writes.
To take advantage of UDMA, both the system and hard drive must support the protocol.
UDMA retains backwards compatibility for previously existing hardware, so the best bet is to buy the latest and finest HDD you can for futureproofing.
In any case HDDs are so cheap these days at a £1 per GB or less that you might as well get the largest you can afford.
For interest, Western Digital hard drives with model numbers ending in JB, such as WD600JB, come with a three rather than one year guarantee, but the penalty is a slightly smaller capacity.