the 4870: 800 stream processors, 256 bit GDDR5 and 512MB (i think i have the specs right)
chip #1 has 1600 SPs, 512 bit of 1GB (this would be the duel chip combined)
chip #2 has 2x 800 SPs, 2x 256 bit of 1GB each (this would be the X2)

technical specs look the same with the exception of the difference in ram size. but in theory id think the first one would perform better since the 2 chips would talk to each other at much better speeds and lower latency (i guess) and would probably be more versatile since it wouldnt have to worry about xfire scalability, it would just be 100% more powerful in any situation, instead of up to 90% in alot.