Listen, for making room for a new os you will have to set a primary partition (for os). So you will need to format the space in your hdd you need for that purpose. You must plan it well thought!
Let say you have a hdd of about 80gb.
You will need at about 15gb for windows 2000 in order to work properly (concerning
and the space you will need for programs also). A 55gb logical partition to save data such as mp3, mpeg etc. so as to to have them stored safely even in case an os problem occurs and for the other 10gb lets say you use 8-9gb for linux and 1-2gb for the swap files and you will be ok, having in mind that you are knew in the linux world and you just want to play with them at first so you will not need a lot of space.
Of course, the fdisk is so old that even the windows 2000 dont support it. So you'd better find a reliable partioning tool or you can use the linux installation program from your linux-cd. In any case, you dont need to buy extra hdd.
I dont know if understood your question fully, but i hope was of some help to you.
Be cool!