Looking for purchasing software recommdendations

I'm in the early stages of researching purchasing management software for my company to help simplify workflow and reduce paper. The market is positively saturated with products in this niche, so I'm looking for recommendations. Here's a bit about my company and the requirements:

-Mid-size (225 employees, $23M in revenue)
-Government contractor
-4 full-time finance/purchasing/AP staff
-Need to track competing bids
-Need to manage purchase approval process
-Need flexible reporting (Crystal would be great)
-SQL Server database back-end
-Web-based user interface preferred

Any tips on products to avoid would be appreciated as well.