If you do want to get something nicer and are willing to spend some money, you need to look for a receiver with pre outs. I haven't looked at how the various receivers work because I have separates for my multichannel system. But you need to confirm that if you use one of the pre outs for your mains that the receiver doesn't automatically shut off the rest of the amp stages for your surrounds and center. To do this cheaply you'll need to utilize the built in amps in the receiver to drive the other discrete channels in the surround setup. You'll also need to invest in a center channel speaker that has the same sound characteristics as your main L+R speakers...timbre matching. If the center isn't matched with your mains, you WILL notice that there is something wrong/different when the sound sweeps from left to right and vice versa. This alone may require you to buy new speakers up front depending on whether there is a center available to match your specific mains. Also you NEED to invest in a LFE sub...the .1 part of 5.1. I didn't know how much of the feel I was missing until I added an LFE to my system. Some of the DVD soundtracks will not have the LFE effects mixed in for the other front main channels. I had set my front speakers to large which allows them to operate at full range because I had a sub tied to my main speakers. I felt I still wasn't getting the full effects. I then got another sub as my dedicated LFE and then everything was revealed as to what I was missing.
As you can see, there are no free lunches. With multichannel systems, think about buying a 2 channel system but expanding the cost into 4 additional channels. Again, I would recommend a all in one system like a home theater in a box as your solution. Many say that this would be more ideal since it is very difficult to have a dual purpose system that is both good with 5.1 and 2 channel stereo music. The set up requirements are a bit different between the two.