nikonx you won't notice a difference.
Consider this. A hard-drive with an ATA/133 interface could transfer data upto 133mb/s. Not even the fastest SCSI 15K RPM drives for desktops can break 100mb/s. A laptop disk at 7200 probably wouldn't go any faster than 50mb/s if you are lucky.
Now, the ram on the other hand is uber-fast. DDR-266 (PC-2100) ram transfers at 2.1GB/s. Since games are always trying to get the most out of system performance, they will never ever under any circumstances resort to using the HD, it is just far too slow. Instead, they will load everything required into system memory and play from there.
A faster HD will speed up loading cutscenes and levels. It is most noticable when booting, or loading office and other applications.
Don't worry, a Ferrari 3200 will devour UT2k4, don't spend extra on an external HD for gaming...