Like you, I am a huge football fan and when it came time to select my HDTV the ability to produce a high quality image without blur, jitter, or other artifacts with the maximum amount of detail during sports broadcasts was paramount. LCD technology is not capable today of producing moving images without blur or jitter unless the TV operates at a refresh rate greater than 60 Hz. Even at 120 HZ, this technology can not compensate for motion blur and live sporting events will overwhelm the 120 Hz video co-processors producing additional artifacts. Also, all HDTVs drop effective resolution with full motion, but a plasma HDTV will retain most of the details (85-90%) over other HDTV technologies like LCD (25-30% for 60 Hz models; 55-60% for 120 Hz models).
With this is mind, I could NOT recommend any LCD HDTV for your application, including the LN-40A550, a 60 Hz LCD HDTV that will blur, jitter and drop over 70% of its resolution during football broadcasts.
The Samsung PN-42A400 or PN-42A450 would be a better choice, but for the same price I would recommend the Panasonic TH-42PX80U for three reasons:
1) The anti-reflective glass of the Panasonic greatly reduces glare and reflections over the weak treatment by Samsung on their plasma screens.
2) The black levels of the comparable Panasonic plasma TV are far superior to that of the Samsung plasma TVs you have selected.
3) I have had burn-in problems with the 42" Samsung 2007 models and have NO indication from the Samsung organization that their 2008 models have corrected this problem. Under the same conditions that produced burn-in on the 2007 Samsung models in my showroom, the 2007 & 2008 Panasonic 42" models have showed NO burn-in. Resistance to burn-in on the Panasonic models is superior to the Samsung models. Burn-in is not a concern with a Panasonic plasma HDTV under normal viewing conditions.
P.S. I own the Panasonic TH-42PX70U and I'm rewarded for that choice every time I watch a football game. There is no more satisfaction than seeing ALL of the details when watching my team win or lose. Being a 49er's fan, I'm certain that this is your priority as well.
Additional Details: Most of your HD sports channels (i.e. Fox, ESPN, ESPN HD2, ESPN HD3) broadcast in 720p because a progressive scan produces 60 true frames per second. More frames means better motion and that is what is most important with sports programming. Next, 720p broadcast always looks best in native resolution. Upconversion to a 1080p screen produces artifacts. Also, the additional burden placed on the video processor by the up-conversion process in conjunction with the burden of sports motion means that the 1080p TV will pixelate or tile the screen more often than the a 720p HDTV.
YOUR BEST CHOICE FOR SPORTS VIEWING IS A 720P PLASMA TV.