Memory is under your control, and if you can keep your content cached will serve you well.
The catch with very small VPS slices for me is disk performance. Small slices means more neighbors, and if a few of your neighbors are high disk use applications your disk latency can get ugly. The nasty bit is you can benchmark and check it all out when you move in, then a month later your site is suddenly a dog.
CPU has the same dynamic, but I think the virtualization platforms do a better job at "fair" for CPU, and if you are using nginx or lighttpd you don't need much so should be treated well by the scheduler.