HotHardware.com posted an ATI Radeon HD 5870 Overclocked Round-up. Here's a bit:

The Recommendation: We know these cards are fast and they all cost a premium over stock cards but you should also consider a few options. First, let's look at different models available within the HD 5870 family. They can be divided into three general groups: reference design cards, overclocked variants, and 2GB models. Stock 5870's are plenty fast and can be had for under $400 at various retailers. Although you can purchase one and overclock it on your own, higher frequencies are not guaranteed (nor supported under warranty), whereas these three Super Overclock, Turbo X, and Lightning models are binned and tested at their given speeds. Also take into consideration the factory overclocked cards in this article command a 25% price premium over the base model, while providing a 11% to 16% performance increase on average.

Next, let's examine the wide range of overclocked 5870's available at the market. There are several to choose from, but unfortunately, we could not get a hold of more for this roundup. Sapphire and XFX currently offer 875 MHz models, while Powercolor just released the PCS++ clocked at 950 MHz. Prices will vary but just remember that it usually is directly proportional to clock speed, since the higher you go in frequency, generally the fewer number of cards will yield that target speed.

Another choice available to you comes in the form of the 5870 with 2GB of memory installed, instead of 1GB. These niche products specifically cater to multi-monitor Eyefinity users, whose systems can utilize the added frame buffer memory. Eyefinity 6 models feature 850 MHz core clocks like the reference cards, but also sport six mini-DisplayPort connections, with 2GB of on-board memory. Our testing showed that on a single display, you would be hard pressed to see any performance gains comparing 1GB and 2GB 5870's.