I use a lot of VST plug-ins, including amp sims. I can't really recommend a product, because, i've never paid for any, and so wouldn't know about value for money (sorry for being a theif, lets not fight). There are some legal free vsts, including a very nice JCM800, which I could find at home. The Ampeg suite/modelling thing is pretty amazing, but, you'd better have a good processor, and I guess it's expensive.
Cubase comes with a couple of amp modelling things, which are under the distortions menu, but have speaker emulation options. I've tried others but I can't remember their names, or if they were any good!
If you want to track the guitars, monitoring through the amp sim, you will need a decent soundcard to create very low latency. I guess the Lexicon is good enough though?