A little 200 would be great for linux. You won't be able to use it as a multimedia box by anymeans, but it would be usefull for a lot of things.
Namely as a kinda nock-around box. You hear about some sort of service like ntp (network time protocal) your kinda interested in. (you would use ntp to syncronise all your PCs in a network, it is kinda important if you do alot of file sharing, because then you would have accurate time stamps and head off some problems that can happen. Also good for buggy x86 hardware that has a different amount of time dift each time you reboot. It's not the coolest example) So you fire it up and putz around with it for awhile. You don't have to worry about frying anything like on your main desktop. You know play around with it edit config files with a axe and sees what happens, intentionally try to crash it, etc etc etc.
Once you get bored with it, just plop a medium sized HD into it and use it for backups, also would be good for a place to upload files and music for when you need to reformat your main machine. Think about it like a big cheap external HD you can mount as just another partition thru nfs or smb (network file sharing (unix stuff) and windows file sharing) Stick a cheap cd burner in it for more backups, or music cds whatever.
I use a old 486-based compac proliant with some old 1.3 gig scsi drives in it for that purpose. Since I got my nice big hd for my desktop 1.3 seems pretty small now, but still it'll take anything that my other 2 computers can thow at it without even flinching.
Of course if you can get a some more memory stuck in that machine the more better off you'll be.