I've used USAepay for the past 9 months and have been very happy with them. It was very easy to setup with 3dcart since they're already compatible with their system. USAepay provides both the merchant account and gateway. I also accept payment via Paypal Standard, but I'd say that 95% of my orders are processed via my merchant account.
Here's a link to their site:
http://usaepay.com/