I think you should think carefully about what you want your receiver to do. If you want new features, then, of course, you must replace it. But what you have is a good receiver, and can still be used for good sound. And a lower model is likely to have a less robust power amplifier section, so if you require your current power levels, you probably need to aim higher. But if you don't need as much power, then it need not be a problem.
I do, however, recommend that you forget about that HK. Read the reviews at Amazon, and you will find that many people seem to have problems with it actually working:
http://www.amazon.com/Harman-Kardon-7-1-Channel-Receiver-Repeater/dp/B0016BGRX6
That Marantz, I believe, is not the latest model, and lacks the new surround formats on BD (Blu-ray). At this point in time, I would be hesitant to buy a receiver without the new formats, unless it were a great deal, like:
http://www.accessories4less.com/make-a-store/item/MARSR6001/Marantz/SR6001-receiver-100w-x-7ch-HDMI-home-theater-surround/1.html
The two downsides to it are that it only has 2 HDMI inputs (which means you will probably need to eventually buy an HDMI switch box; this is worse than you might think, because HDMI devices can be finicky) and you need to do the new format decoding in your BD (Blu-ray) player (which can be sent as multichannel PCM via HDMI). Decoding in the player that way is fine as far as sound quality goes.
You can currently get free shipping on Marantz products at that site; see main page, where I believe it will tell you to simply enter the code FREESHIP when ordering.
For a new model with the new formats built in, I would recommend looking into the Denon AVR-790:
http://www.usa.denon.com/ProductDetails/4891.asp
If they have implemented the video chip properly, it will have stellar video upconversion. See:
http://www.audioholics.com/buying-guides/how-to-shop/2009-500-receiver-comparison-guide
It retails for about $500 as you can see.