"Exactly how much is that?" Good question, no answer. I recommend you get some sort of a network byte counter, then go to a few sites that feel similar in size/scope to what you plan. Use their "bytes per visit" as a ballpark figure.
For instance, I'm using Safari and turned on the "Show Network Timeline" then reloaded hacker news. 34kB. Then I went to slashdot.org: 643KB for a first visit, probably more like 74KB for subsequent front-page load (scripts are the bulk but they cache).
If you use a VPS solution, you will be managing your machine which is nice, but if you aren't already skilled at that it will be a distraction from your webapplication that you might be better without.
The big question with VPS solutions is "how fast is my machine today?" I have a tiny slice at VPSlink that used to be nicely snappy, but now is painfully slow. Something outside my visibility has changed on my server and there is nothing I can do. My slightly cheaper server at RapidXen is much faster.
Slicehost guarantees CPU share based on your slice size, but I don't see the denominator, I assume since a nearly 16GB slice is available that is the denominator which would give you 1/64th of the machine, worst case. Disk IO is another thing entirely, 1/16th of a modern CPU is ok, 1/64th of a disk drive spindle is painful. I suspect that is why my VPSlink machine has become slow.
Money can mitigate some of this, just buy a bigger slice. Mine are tiny because I am interested in how much I can do in a ~$7/mo server.