That's a tough one! I think it's your call there. Does the newer Marantz have any features the others don't that you will need in the near future? The Marantz 8500 is one amazing receiver, so for arguments sake, I did the following:
I was comparing the Marantz 7500 to the 8500 and the major differences were the HDCD decoder, the HDMI inputs and outputs, a copper chassis, 20 more watts, and a toroidal transformer in the 8500.
The 8400 has the toroidal transformer, copper chassis, discrete amplifiers, and 5 watts over the 7500. The 7500 has no HDMI inputs or outputs, nor does it have a toroidal transformer. My vote goes to the 8400 out of the two.
The Yamaha RXV2500 has the most watts at 130x7, discrete amplification, YPAO speaker setup, and Clint's thumbs up on one of the best values under $1000 for any receiver. Yamaha's website features Clint's review 1st. Here it is:
www.yamaha.com/yec/products/reviews/pdf_reviews/RXV2500_review.pdf
Now, if you have access to ubid and can pick up the Marantz 8400 for $550 or under, my vote goes to the Marantz 8400. It has a 3 year mfg. warranty which can't be beat. If you are only talking a $50 difference, I would "consider" the Yamaha. Hope that helps.