If you are getting download speeds of 3-4 megs per second, your Internet connection is not slow. Perhaps there is something else slowing down your experience, such as spyware or low memory. If you are running on Windows XP or 2000, download Microsoft Windows Anti-Spyware here. If you're not using XP or 2000, then use Ad-Aware. Download it here. Both of these anti-spyware applications are free, and Microsoft's is unfortunately the best out there right now. You should also consider if you have enough RAM installed on your computer, and how many programs load up automatically when you start up you computer, eating your ram.
DSL is kind of hit or miss. There are really good DSL connections out there, and really crappy ones. Usually, they lock you into a contract, so know what speeds they are promising you before you commit. Any free digital camera is probably not that good! Personally, I have been switching back and forth every six months from one high speed connection to another, just because these companies like to play that game of doubling your price after a period of time. I have had the best success with cable Internet, and, as I said, 3-4 megs per second is not slow at all, athough it could be faster. DSL most likely will not beat the speeds you are getting now.
As far as anti-virus, the best free one by far is AVAST! download that here. It is probably a bit better to be running Norton or McAfee, if you have the $40/yr. or so to spend. I'm not sure what firewall is on your DLink router, but all you really need if you are running XP is to make sure you have downloaded servicepak 2, which includes Microsoft's firewall. Otherwise, the old standby free firewall is ZoneAlarm available here. Yes, these days if you are on broadband, you need all three: Anti-Spyware, Anti-Virus, and Firewall. I hope this information helps. If you have more questions, PM me.
-pro