Is this a good graphics card?

Crossfire? but that card's not in the same family as your 4800.

7850/7870 > R7 260x

By a decent bit. The R7 260X is a rebadged 7790

Abandon the idea of crossfire with a 4800.

Besides nothing in the 7xxx or R9 series being able to crossfire with a 4800, crossfire causes the higher card to perform at the level of the lower card. Instead of getting performance near an R9 260 + 4800, you would only get the performance of 4800 + 4800, which is lower than a mid-range 7xxx by itself.

Also consider that crossfire, just like SLI, is not going to give 2x performance. Count on at least an 80% performance boost in games that run well with multiple GPUs, but also count on a lower minimum fps and slightly increased split-second FPS drops.

1 card = 100%

2 cards = 180%

3 cards = 240%

4 cards = 270%

With every additional card in a multi-GPU setup, price vs performance degrades. These numbers aren't real-world, just hypothetical to show how things progress. In most situations, stick with single GPU. It is increasingly rare to see multi-GPU beat single top-end cards on both price and performance at the same time.

If you can afford a single card that gives nearly 2x the performance of a single 4800, get the single GPU and sell that old 4800. Someone out there always wants another AMD for bitcoin mining.

EDIT: I just looked into things. The R9 260X you're considering is capable of delivering more FPS than two 4890s. I'd still go with a 7790 if you want to stay near the 260X price, but you'd be better served by waiting for a 7870 GHz Edition for around 40 USD more than a 260X or a 7790. It tends to benchmark a lot higher for the money than either budget card.

Yes, that's correct.

Consider the R9 series if you play games that take up a lot of VRAM. The R9 series tend to have more VRAM and higher transfer rates than the cards they're based on.

Also consider that the R9 series doesn't need to use a crossfire bridge, instead sending all communication between each card across the PCIe channels. Even PCIe 2.0 8x doesn't get saturated by three way crossfire. I've heard four way does fine as well.