Q's on SLI config.

I have now one xfx 7600gt 256MB. overclocked at (core 631MHz. Memory at 1.6GHz. default is (core 590 memory 1600)

my Q's are well i have heard that Sli will increase proformace by 80%.???
i have also heard that running one fast card is still faster then running two that are alittle slower.??
I also heard that two xfx7600gt will run about as good as one 7800gt.??? that's not much better nor is it near 80% better as the claim.
well the 7800gt defaults are lower core clock and lower memory clock (just that it has 24 pixel pipelines) as for the 7600gt has only 12 pipelines.
Will i be able to overclock the two SLi cards higher as in higher core speeds or it stays the same???.
last Q's is since my 7600gt is 256MB memory (would having two in SLi both being 256MB. be like having 512MB memory ????
like as it is with ram system memory (2 x 256MB = 512 ram.)

FULL comp Specs

mother-board ASUS A8N-sli Deluxe
amd 64 4000+ overclocked at 2640mhz 1G cashe
xfx7600 Gt overclocked (core 631 Mhz) (memory 1.60Ghz)
2 x 512mb Corsair dual channal (overclocked as well)
2x 80G sata 7600 rpm hard drives in RAID zero. thinking of upgrading to two WD Raptors 10,000 rpm in raid zero

i am getting somewhat the idea on the first three q's i have asked about on other posts.

BUT still the last Q's on having two 256mb cards running in SLi if it would then give me 512mb as it would be true for ram memory, is still a Q's needed. Thanks