The Denon AVR-5803 is an exceptional piece of hardware, one that has yet to be beaten in terms of sound quality, atleast as far as A/V Recievers are concerned. Although the AVR-5803 may not have all the latest technologies, eg. Firewire, HDMI, Dolby Pro Logic II, Dolby Headphone, Upscaling etc. it is still the one of the best, if not the best, sounding A/V reciever there is. So, unless the missing new technologies are and issue for you I would say most definately go in for the AVR-5803. You will not be dissapointed.
PS: The remote included with the AVR-5803 does leave a lot to be desired, but since I personally have only used my Philips Pronto from the very begining, I can't comment too much on it.
I used to own a Denon 5800. The stereo performance was pretty good, but it didn't do anything for me when it came to movies. You may want to look into entry level separates for close to the same price. In my opinion, you will get better performance.