hi, how many gigabytes should I allocate for linux? This goes for in case I don't like one linux flavour, then I install another one. So, 5GB partition would be fine?

I'd recommend more like 20gb, if you have the space. Linux distributions usually come with lots of applications (this can vary significantly), and if you want a complete install, then you MAY need this much space. 5 gb will generally work if you do a standard "desktop installation", in which case not all included applications will be installed (they can be installed individually later, however). Don't forget about a swap partition, as well, if you have less than 512 Meg RAM or so. If you only have 128 Meg, for instance, you will probably want a 128 or 256 Meg (1/4 gig) partition. If you have 512 Meg or more, you MAY not need a swap partition (I have 512 and don't use one, but it appears necessary on my 128 Meg machine). Good luck.

Dont forget to activate your designated partition if you dont want to use a boot disk and have an auto install from the hard druive--do this using FDISK

I agree with C_Legend, give as much as you can....even better if you can dedicate a whole hard drive to GNU/Linux. (Not to sure about NOT using a swap partition though, but then again if you have the RAM...?)