(the levels of advice on these forums astound me - no offence, but I wish people would put more effort into helping newcomers out - sureley if you cannot post a reasonable reply, is there any point in just posting a one or two liner? Just a waste of bandwidth, button clicking and traffic IMO.
anyway................
Firstly, before thinking about adding additional storage, take in to account how you use your PC. If you have nearly filled an 80GB drive, then its safe to assume you do no housekeeping (by that I mean regulary deleted unused files, arhiving data etc) and simply adding more storage will encourage you to be more lazy with the way you work.
Various methods can be used to free space on a hard drive, and also bare in mind that due to the inneficient way Windows Indexing works (if you dont switch it off) the more files you store and access, the more footprints are left in the registry, the more you degrade Windows performance, and it will all go horribly wrong, usually ending up with you losing vital data.
Make a point of regulary cleaning tmp and cached files as these accumalate over time and take up disk space. You can run the command prompt and using wildcard strings and issue del c:\*.tmp -f /s which will delete temp files from your hard drive and also force deletion from all subfolders. PLEASE BE CAREFUL with this command. You can also change the file extension to the most commonly used uneeded files such as .bak, etc etc
If you feel however, you require more space, then installing an internal HDD is not the headache you may imagine it to be, and once installed, will display as a drive in 'My computer' allowing you to drag & drop from one drive to another. An external unit works in exactly the same way, but of course, without the need to open up your case, and fiddle with SATA/IDE cables, jumper switch selection and BIOS configuration etc.
File access speed is slightly slower over USB2/USB and theres usually a little more latency in an external drive then internal. Remember your adding an long USB cable, connection interface and other electronics which are all impeding speed against an internally installed one that just has to push data down a multi channel high speed IDE/SATA cable.
however, for most users, HDD speed and access times is not an issue these days.
Be aware also that as a beginner, if you elect to install an internal drive, read the instructions. If you opt for the latter, an external unit (which would be better in your case) then you can just plug and play.
Opt for a recognised brand, the higher quality the better, makes such as Buffalo, Linksys, Snap, Freecom all do quality units.
Hope that helps