First, if you want better versions of the VSC stuff and/or a good GM module then you should look at Edirol products (offshoot of Roland) here: http://www.edirol.com/products/audio_index.html
Personally, the only GM-compatible softsynth I've found that I like better than the Roland VSC-series stuff is the Yamaha S-YXG50 XG softsynth that can be found here: http://www.yamaha.co.uk/xg/html/midplug/m_mid8.htm
Either of those solutions will work and demos are available for you to try before you buy - which is important because GM softsynths are highly susceptable to latency (a delay between hitting a note on a midi controller and hearing the note) so using them for real-time sequencing can be a problem. For this reason it's often better to use a GM hardware module for sequencing, although you can use a softsynth for playback. One thing that can help reduce laptop audio latency is a freeware program called ASIO4ALL, found here: http://michael.tippach.bei.t-online.de/asio4all/
I switched to XG from GM (better sounds and control IMHO though Yamaha is slowly backing away from this format seemingly) and use an XG-compatible keyboard for sequencing, then use a laptop with the Yamaha S-YXG50 softsynth for playback. I use my laptop when I play an acoustic piano to give me SMF backing.