this could be a long debate, but after a lot of research and own experiences i TRY to make this short

[Edit: Nice 'try

]
The most important question here is which OS you install.
If you install XP, then the best thing is actually NOT to partition.
XP has 'built in' filesystem optimizations like pre-fetch and 'self organizing of file structure' which do NOT work across partitions. (keyword: layout.ini).
You can do a google search and/or read a lot abut that online..also...MS for this reason recommends that XP (under NTFS) only uses ONE partition.
HOWEVER - if you have strict 'DATA', eg. videos/pictures...or backups, or archives, zip-files, downloads....eg. files you do not use a boot-time and where access-times dot matter then you can create a second partition and put these data on it - if you WANT.
HOWEVER - also for this (inmy opinion) there is really no need. I had several partitions years ago, eg. one partition "D" for "data". But you can also create a FOLDER data on the physical drive C: and put shortcuts to it.
Instead of woring with a second partition D: then you just do the same thing using C:\Data whic does not make a difference in real 'life', working with it. With the advantage that the XP internal optimizations then work - eg. if you put a game on C:\data (instead of another partition) it can be optimized.
(Optimized meaning windows defragger puts the file(s) 'in front' of the HDinstead of yuo being stuck on the slower second partition. HDs get slower on the 'end'..meaning if your new partition (after C: ) is far behind, say 50 gig or so, its slower than the partition 'in front'.
Als..i my opinion its BAD to have a small partition for the OS (eg. 20 gig)..again...best would be that ALL your OS *and* often usedfiles/apps/programs reside on ONE partition.
You wil have bigger defrag-times with bigger partitios (of course) - but overall i think its better in conjunction with a good defragger, eg perfect disk if access times etc. matter to you.