Increasing memory on my Dell desktop

I have a D
ell Dimension 4550 desktop which I purchased in 2003. It has been relatively trouble free, and about two years ago I replaced the factory memory module with a 1 gig module, and the machine was noticeably faster, especially on startup. It's beginning to bog down again, but my owner's manual says 1 gig is max for this machine. Is there an inexpensive and easy way around this restriction? I'm assuming if I add a higher memory capacity my machine will respond more quickly.

Thanks in advance for any suggestions.

Dean S.