Western Digital are good as are G-Tech. Capacity is one thing but the drives technology is very important. You want something with minimum 32mb cache, read/write speeds that match your internal drive.
320GB is relatively small in terms of size these days, your computers HD shouldn't really dictate the size of the external HD, instead I would think about what will you use an external for and how long will it take you to fill it up.
The Western Digital 'My Passport Studio' will offer you the fastest transfer rates with the FW800 connection. Desktops wise the 'My Book Studio' or 'My Book Studio LX' these also offer FW800. USB 2.0 bit dated now, invest in something that's not obsolete. That's the WD breakdown.
G-Tech offer nice drives Portable or Desktop like the G-Drive, G-Drive Mini, Mobile. If you have a bigger budget look at the G-Safe.
There really are may options, the main thing to remember is to look beyond the brand and size and see what is under the hood.
My experience with Lacie is horrendous by the way, cheap horrible drives IMO.