OS X is very good about RAM. We have quite a few G3 iMac's that are 333mhz that we run 10.4 on here with 256MB of RAM. We've thrown 10.5 on some 1.25ghz G4's with 512MB and it's run fine for normal useage.
Is this a web/email/office machine? If so, you'll probably be fine with 512MB of RAM. Startup time may be a little slow but once it's up it won't be that bad.
Having said that, RAM is awfully cheap these days...
Do you have another Mac? Like a new iMac or MacBook/MacBook Pro? If so, maybe you might consider maxing out your everyday work machine and then giving that RAM to the Mini. Like if you have a MacBook with 1GB of RAM, order 2 or 4 for it (depending on your MacBook revision) and then give the Mini the 2 512's from your MacBook.
Whatever you do, keep in mind that the Mini will come with 2x256, so plan accordingly.