The only real difference with the receivers you're considering versus your 2800 is the acoustic room calibration function that comes with those models. You're getting only a minor upgrade in the amplification, and the rest of the improvements are convenience features like component video switching and multiroom amplification. If this is worthwhile to you, then by all means go for it, but IMO the receiver that you already got is more than sufficient to drive your speakers. If you choose to get a new receiver, it will basically be for the automatic room calibration, the convenience features, and the more advanced digital decoding. How much better those receivers will drive your speakers will probably make an inconsequential difference, because the Paradigm Monitor series is not especially straining on amps. However, you might find that the calibration feature is essential, especially if you have odd room acoustics and an asymmetrical speaker setup. Whether it's worth all that cash is up to you.

Between the Yamaha and the Denon, on paper at least I'm not sold that the 3805 is worth $400 more than the 1400. I've only briefly tried out the 1400 and have yet to see the 3805 so I can't tell you anything definitive about either one. After trying them out for yourself, you might come to a different conclusion.

If you wind up opting for the HTR-5790 (haven't seen it in stores yet), keep in mind that Yamaha has previously removed a feature or two from the HTR versions of their midlevel receivers. Make sure that the unit you buy has two things -- 1) the YPAO acoustic room calibration feature; and 2) preouts for all channels (the previous HTR-5590 inexplicably did not include them, even though the equivalent RX-V1300 and lower priced HTR-5660 DID include them).