It's tough to have a car that you never drive and have a chunk of change tied up in. I think about selling my car occasionally, too. I always come to the same conclusion:
1) I would really miss the dreaming, researching, planning, installing, repairing, replacing and driving.
2) I wouldn't get nearly enough $ for it
3) I would inevitably end up replacing it with a more expensive/problematic/time-consuming project car in the not-so-distant future (FD + LS1 = drool).
I think the best way to escape the 'modded but unused car' rut, is to get out there and use it. Commit to getting together with other car nuts and racing it (autoX, road course, drag, whatever) -- drive the car the way it was meant to be driven.
I don't know about you, but a heavily modified Sentra sucks to 'cruise' around in: stiff ride, no radio, peaky engine, loud, lousy paint etc. It's even worse if you have a nice, newer daily driver. On the other hand, hammering the sh*t out of it in a controlled environment is a blast.
I like the Optimas a lot but have heard a lot of bad stuff about quality these past few years.
I have a trunk-mounted Optima Red Top with cut-off switch. I parked the car in late November, turned the cut-off switch to OFF position. Nice weather is back and road salt is gone, so I've put it back on the road. As part of the prep I threw a charger on the Optima expecting it would need a charge -- it read fully charged! Sure enough, it cranked right over. Great battery!