There is no 7890, AMD have just released the 7870 Tahiti LE though, which is a faster 7870 that is closer to the 7950 performance wise. Similar to what Nvidia did with the GTX 560 448 core and the 570.
You can also crossfire the 7870 LE with any of the 79xx series cards as well, so if you already have a 7950/70 and can't quite afford another one the 7870 LE could be a good option.
They will call it the 7870XT (tahiti) rather than the 7890. It uses the same Tahiti architecture as the 7950 and 7970, but on a 256 bit platform rather than 384.
well I'm going to go for the XT because its a first time build its faster than the regular 7870 and from aria it comes with games witch i don't have many of at the moment.
On the note of performance, techpowerup reviewed the card a couple weeks ago: Review link
Overall it's notably faster than the HD7870, a little under 10%.