As soon as you say FCS, I say the 20" 256MB VRAM. You can never upgrade the VRAM, and you'll appreciate it in Motion and Shake (and possibly the next version of Final Cut, you never know).
System RAM is relatively cheap and will only get cheaper. You can upgrade to 1.5GB dual channel for $150 in a few months when you have the money. Then go to 3 or 4GB if/when your projects get more demanding.