VPS Server Stats -- how much RAM do I need?

I realize this topic has been covered a lot, but this should be quick and easy.

We've got a WPMU install with about 100 active blogs, and we're planning on growing to 250 blogs this year. Currently, we're hosted on a shared hosting plan and need to upgrade, but can't afford a dedicated server yet.

(Using about 4GB of disk space and about 30 GB Bandwith/month)

I have a friend who owns a hosting company who is willing to work with me to create a custom VPS with Nginx & memcached, and he's given me the following two options. He said that I can customize these choices (i.e. add more RAM) if necessary.

(Also, we're going to implement the W3 Total Cache with Amazon S3 or CloudFront CDN to offload theme & media files)

Option 1: Advanced Linux CentOS 5 - $99.50/mo or $272.85/quarter, or $960.00/year and will have 50GB of disk space and 3000Gb of data transfer per month, 4GB RAM.

Option 2: Premium Linux CentOS 5 - $59.50/mo or $164.85/quarter or $580/year and it will have 20GB of disk space and bandwidth 1000Gb data transfer, 2GB RAM.
Here's my question... If I go with the cheaper option for now, is 2GB RAM enough? Should I ask for 4GB or 8GB or more?