Borrowed from HIS HD5870 page
"500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireXTM technology in dual mode)" http://www.hisdigital.com/us/product2-520.shtml
It should just about work on a good quality 650W like yours, but as already mentioned, you will need 2 x PCI-E 6 pin power splitter cables or 2 Molex to PCI-E 6 pin power converters (probably better). Having a good quality PSU is often more important than the actual maximum advertised output as cheap PSUs often just don't cut the mustard when compared to a good quality 650W PSU like yours even if it's advertised as 850W plus they sometimes don't provide enough power on each rail, also it's simply not worth risking such expensive components with a cheap low quality PSU that is much more likely to power spike Etc while generally providing a less stable supply which is more likely to cause future PC problems.
Your 650W PSU will be worked hard however when the PC is pushed and in the long term I would look at saving for a quality 850W PSU, preferably with 4 dedicated PCI-E 6 pin power adapters, this will also leave room for further future expansion. Each Ati Radeon 5870 can draw a maximum of 188W even though they use very little when idle. Work it out, 2 x 188W = 376W, this only leaves 274W left for the rest of your PC from a 650W PSU. Out of 274W left, your I5 750 uses a maximum of 95W which leaves 179W left for your motherboard, memory, cooling fans, hard drive(s), optical drive(s), USB devices, lights and any other devices. Obviously if you overclock the power consumption could be slightly higher still.
You specify you have an Intel I5 750, most Intel chipsets don't support Crossfire (multiple Ati GPUs) and support SLI (multiple Nvidia GPUs) instead, but there are some exceptions and a few motherboards even support both, please check that your motherboard certainly supports Crossfire before purchasing another Ati Radeon 5870. If not, you will either need to stick to a single GPU, or you'll have to sell your Ati Radeon 5870 card for Nvidia cards to run in SLI instead. Incidentally, things are hopefully changing for the better soon to allow people to mix and match multiple graphics cards from different vendors using Crosslink technology and most new games already support it with a new driver, but at the current time only one graphics card is available with a crosslink bridge built in and that's the mid-range Ati-Radeon 5770, although plans to release numerous cards with the new technology are imminent. A Crosslink graphics card can be placed in any free PCI-E slot on any motherboard with any other graphics card installed from any vendor, out of 2 graphics cards only 1 needs to have a Crosslink bridge built in, out of 3 only 2 need to have a Crosslink bridge and so on. There's also currently one motherboard that I know of that has Crosslink technology built into it where any standard PCI-E x16 graphics cards from any vendor can be mixed and matched, that's the Crosshair IV Extreme board with an AMD chipset, but more are likely to follow. Eventually being forced to stick to either Nvidia SLI or Ati Crossfire to utilise the power of more than one GPU will hopefully be a thing of the past.
PS: I think the AX series are fully modular and are the most efficient while the HX series are modular except for the 24pin and CPU power, while the TX series are non modular, but don't quote me on this. Even if you buy a cheaper Corsair PSU, they're all good quality as long as they're SLI certified (or recommended for Crossfire).
You specify you have an Intel I5 750, most Intel chipsets don't support Crossfire (multiple Ati GPUs) and support SLI (multiple Nvidia GPUs) instead, but there are some exceptions and a few motherboards even support both...
I think you'll find it's the other way around - CrossFire doesn't require a license from AMD so it's on all multi-GPU capable boards. SLI however does require a license from Nvidia, so you don't get SLI on budget boards.
Gigabyte, for example: GA-P55-USB3 only supports CrossFire, GA-P55A-UD4 supports CrossFire and SLI
Asus: P7P55D-E is CrossFire only, P7P55D-E Pro supports both
AFAIK, it's only ever been Nvidia chipsets that are SLI-only.