I am seeing data rates now fluctuating between 54mbps-121 mbps for the WGA600N (connected to my DirecTV receiver) as reported by the D-Link Wireless Status page. Is that ok.?
If everything that you want to do is working, then it's okay.
I was able to stabilize the signal strength on the 5 ghz to 40s-50s.
So here's the skinny on signal strength -- it's probably a ratio SIGNAL/NOISE and not really a signal strength meter. So if you're having problems -AND- you're getting bad levels and nothing you can do to improve the strength of the signal is available, consider reducing the noise sources. Look for sources that might be introducing noise: a poor power strip, fluorescent lights or touch-lamps, fan motors, and (unfortunately) many kinds of computer and AV equipment.
Note: if all you have are iffy levels but everything is working fine, then don't fix it -- it's working fine. It's an indicator, not a problem.
I wound up changing the fixed speed on the D-Link back to auto (best) because I kept having "can not access content" errors. Maybe it is best to leave it with some regulation.
The data rate is designed to float and react to various conditions.
Do you think I should try to use the 2.4 ghz band? I read that some people get better results with 2.4 surprisingly. My other concern is that the adapter is basically next to the microwave where I read 2.4 is more succeptable. 
Well I don't think you have a real problem to solve. If you don't really have one, then leave it alone.
That said, both 2.4 GHz and 5.0 GHz are going to have reflections causing multi-path distortion in the signal in places -- to a laptop they'll seem like hot and cold spots that come and go based on position and direction. If all this stuff is in a static location (e.g. not a laptop) and just isn't working, then a band switch still might work because the wavelengths for 2.4 and 5 GHz are different -- the hot and cold spots will change.
But if you can't change bands, try changing positions -- like you learned from the 5 GHz thing, placement is everything. It really doesn't have to be a low to high change, it may simply be a few inches right, left, back, or forward that can change everything.
Is it true that if you have g devices on the network that you will not get better than g speed even for N devices? Should I try 2.4 instead (even though I have other g only devices)?
Partly.
Your N gear will still take decent advantage of some of the improvements in the spec like frame aggregation and MIMO decoding, even if the AP is servicing B and G clients. This means your N cards will still be faster and work farther than their 802.11g counterparts in the same network.
Because of the presence of ANY incompatible signal (including signals not coming from your particular network) prevents the continuous use of some 802.11n features by your AP and clients, some of the substantial improvements available in an N-only network just won't appear constantly (or even very often), especially on the 2.4 GHz band where you simply can't get away from such signals. But other N improvements that won't interfere with legacy networks (or vice-versa) will still work when both the client and the AP are 802.11N.
Yes. Try 2.4 GHz as part of solving a problem.
In short, are 54-121 mbps adequate N speeds?
Adequate is really up to your particular need.