Yes you can stack two (or a maximum of 16) 3508s by using the gig-ports on the front. You can channel multiple ports to support 2 Gig connections between two switches. You can use Gigastack GBIC's (copper) or even Multimode Fiber links (You'll need the GBIC's and the fiber patch cables)
For the future, depending on the size of the organisation and the growth, it is best to go with the 6000 or 6500 series, the 5000 series is going to be taken off market in the future...
Good luck!
Regards,
Coen