I'd forget realistic when it comes to VST amps. It's more about how good can they sound. I think the consensus is that the free LePou and TSE amp sims are better than any of the paid for programmes out there. I'd agree with that, too.
Excuse the shameless plug, but this clip shows off reasonably well some of the free VSTs. This was done with the TSE X30 (Engl Pball sim) and a TSE 808 (808 sim):
[VIDEO]http://www.youtube.com/watch?v=M9fq7kJ2BLs]Like The Average Stalker - Soilwork Cover - TSE X30 - YouTube[/VIDEO]