Which CUDA video card should I use for encoding video?

Hi, I have an old (Feb. 2007) liquid cooled (cpu+2gpu)Digital Storm Twister with a Q6700 OC'd to 3.2GHz, 4G of DRAM(and the original 2G ready to add back in if a good idea),evga 680i, 850w thermaltake and two XFX 8800GTX in SLI. Actually one 8800 is working(or one slot is). And the health of that one is suspect because of BSOD's, particularly on wake up. Vista 32 bit usually blames the nvidea driver which I keep clean installing and updating.
Anyway, I think I should get a single air cooled card to take advantage of DGDecNV CUDA software as enabled in BD Rebuilder. My question is how many CUDA cores, i.e. which nvidea card, is the sweet spot? 560 with 448 cores caught my imagination at $289, but 550ti at $119 made more financial sense for a classic PC. Or is 550ti a waste, bottle-necked with this system(for CUDA considerations only). Or would the CUDA advantage of a top of the line hydrocooled evga astound me?
BTW is there a rule of thumb to compare how many CUDA cores would work about like an extra CPU core?
Thanks to all with the patience to read!
B