I'd second Daniel's suggestion. If you only had a 2.0 setup, I'd say stick with your Sony if you're happy with it, but you have the full compliment of speakers for a 5.1 setup, so I'd definitely take advantage of that with a newer receiver that has Dolby Digital, DTS, etc. Almost every modern AVR has DD/DTS, unless it's a stereo receiver.

Pro Logic and Pro Logic II are great for 2-channel sources (VHS, most TV, etc.), but I'd definitely look for something that supports 5.1, especially since you already have the speakers.