You will hold a windows partition on a 30gb SSD never mind a 256GB. I would go for a 64GB just for that extra storage.
How much Windows related stuff are you going to install?! I think Windows 7 by itself takes up about 30GB.
As long as you're happy with less than optimum 3D performance. It all depends on what you need a windows install for and whether it's you're only machine. On my MBP I was running Win7 in a vmware vm but I never needed it when on the road and when at home it was just quicker to do everything if I just used my i7 rig - in most cases a lot quicker. So this week I reclaimed 20GB of drive space by deleting the VM, I'm not going to miss it.
Loool... Windows 7 including Adobe CS + all media-players and web-software is some 15-20GB only. If you want to install OSX and Win7 on your SSD and have another drive for storage (external drive for videos and other big files) then a 128GB SSD is plenty enough allready. Grab a 1TB external FireWire 800 (IEE 1394b) HDD in addition to the 128GB SSD and be done with it. EDIT: An external drive for storing your files has the positive thing about itself, that you can use it on with every other PC aswell instead of having duplicates of all your files on every machine itself. I'm using a NAS/external HDD for several years allready because of this.