Ant-Spam recommendations / warnings

I'm purchasing an anti-spam product and would appreciate some of your thoughts on what products you like (and don't like).

I'm looking for a gateway server to catch spam before it hits my server. Anything that runs on the mail server itself is out. I'd also like it to block viruses & certain attachments before they hit the server. There's a couple of other requirements, but that's the big ones.

Any ideas on what products and/or technology to use?

Some products I'm looking at are DynaComm, SurfControl, Intellireach, and Proofpoint. Any thoughts are appreciated.