Seagate all the way... I have five Seagate Agent Pro (4x 500GB, 1x 320GB) on my Windows Home Server, plus three Seagate Agent Go (1x 160GB at home, 2x 500GB in each of the Geekzone servers for backup). Those work really well.
I tried the WD but found out that it goes to sleep after 15 minutes, disconnecting from your PC - and the only way to bring it back is by rebooting your machine. Not even unplugging and plugging would start it again.
I'd go the seagate way as well, not much difference in price, and with the prices of hard drive and enclosure these days you'll probably save between $20.00 to $40.00 and you have to put that in yourself and if not careful you may or may not break a pin :)
>A little hard to fit another HDD in my Dell XPS M1330
Well... little yes. I jut swapped out the DVD-ROM with a HDD caddy to have a second HDD in my laptop.. to run VMs from. Speeds things up nicely. I treat my laptop as a portable desktop more than laptop.