You will gain performance for sure, because you are adding another card, however, it will slow the faster card down to the slower card's speed. I'd suggest just getting another 4870 if it's cheaper. But yeah, they will work together, because they run off the same driver package, but it's not ideal to mix match chipsets and brands as well. For best stability, you want to pair the exact same cards together.
I can't seem to find any new Palit Radeon HD 4870 Sonic Dual Edition 512Mb. That's what I have. I should probably consider looking for a used card instead. I just saw there's a Palit ATI Radeon 4870X2 4870 X2 2GB Dual GPU for sale on Ebay. That could be interresting...
No, the HD4870 and HD4890 are different architecturally.
This is false. They will Crossfire, but the 4890 will slow down to the clock speeds of the 4870. Also, 512MBs of the 4890's memory will be lost. Only the amount equal to that the other card will be used.
6 months ago, any other 4870 512 would have been an easy match-up because they were selling for $120-$140. Now, they hardly exist at all, and the few that are around cost $170 or more. The same is true of 4890's, with their prices in the $210-$300 range. For that money, it only makes sense to upgrade to the new 5830 or 5850, which would smoke either of the 4800-series cards in a single card match-up.
The main reason you can't find any Palit ATI cards is because Palit is now nVidia-exclusive.
You mentioned eBaY... Try to find a solid used 4870 there at a reasonable price. That or upgrade to the 5800-series cards.