Okay, thanks! So you have experience with Slicehost? Moving up to larger slices is a painless task?
My understanding is that Xen is pretty tricky admin, but you can do your own ubuntu or CentOS VM under vmware ,give it x Meg RAM, throw ab or jmeter or trample at your app, see what happens.
My one bump up was a tiny, almost vanilla image and took even less time.
And, if continuity of service is important, it's possible to launch the larger clone first, then do a near-instantaneous cutover in various ways (esp. if your site can be read-only for a short window, and you can forward hits from the old machine if DNS takes a while to update).