A couple weeks ago I just did what you'd like to do.
I used Partition Magic 8 to set aside 2 Linux extents, a 5.5GB main extent and
a .5GB swap extent.
Then I installed Mandrake 9.2, which recognized the existing partitions(extents)
and the install went very well. I can use MOST of the features in Linux, except
the modem(winmodem) and USB ports. But I'm new and will learn alot working
on these problems. Make sure to choose Linux as the default OS, or Lilo won't
work right in the WinXP boot record.
Since I can only use my WinXP partition to get online, I later created a 2 GB
FAT32 partiton, which can be accessed by BOTH WinXP AND Linux. So now I
can do word processing on Linux, save to the FAT32 partition, and then use
WinXP to email documents, or read/edit them with Word. The conversion
from KWrite to Word .doc format might not be perfect, but .txt works well, so
I installed ALOT of stuff in the Linux partition, because I want to try Gnome,
KDE, and many of the browsers, features, etc. Once I know which apps I like
my next Linux install will be smaller and more efficient, and contain just what
I like. Right now the actual Linux, with MANY apps totals 2 GB, leaving about
4GB for data, etc. That's actually as much space as the computer I just
'retired' had on it (6GB total). My new computer has 40GB, of which I have
about 8 GB for Linux, including the 'common' 2GB FAT32 partition. WinXP
takes up about 2GB also, leaving 30GB for WinXP data.
You'll be OK using 6-10GB for Linux, unless it becomes your primary OS. But
for a 'test drive' you'll be fine.