I have now a single sapphire hd5850 which is old and have only 1GB memory, I found a great deal on the sapphire Vapor-X hd5850 which have 2GB memory, will it be compatible with old hd5850 on crossfire???
More abouthd5850 crossfire compatibality
yes, they will still work together. the extra gb of memory will be of no real use though, so unless its cheaper than a 1gb version its probably not worth it.
^+1 every benchmark i've read shows about a 3% performance improvement at best with 2gb not worth it imo.