Yes, the HD 4850 is an ATI card. Most likely you'll find it listed as Sapphire or Gigabyte or HIS or Visiontek. Those are ATI partners who actually manufacture the card based on ATI's designs and chips.
It's a good gaming box. There are better video cards and CPUs (GTX 260 Core 216/HD 4870, Q9550), but they cost more and give you less performance for the money. The HD 4850 can give you 80 fps in COD at 1680x1050, or 30 in Crysis, or 41 in Oblivion. If you add a second one it will gain between 30% and 80%, depending on game.
I've never heard of Arctic Power. That in itself is a pretty bad thing. Not that I'm a PSU expert or anything, but still, it worries me a bit. Does that PSU have 2 PCI-E connectors with 6-pins each? You need one such connector for each card.
The Q9300 can be overclocked, but it's bad at it. Multiplier 7.5 doesn't help at all. For example Q6600 has 10 and Q9550 has 8.5. The Q9300 also has a smaller cache than both.
That Arctic Power is OK for the price, but it's not honestly labeled. That should not be called a 700W PSU. It has two rails with 22A each, so you can expect at best 44A, more likely 35 to 40. Compare that with a Corsair 650W that claims only 650W but delivers 52A,i.e. way more than the so-called 700W from Arctic Power. Also, the Arctci Power comes with a single 6-pin connector, so you'd have a problem adding the second video card (assuming you get HD 4850 cards, which needs a single 6-pin connector each). Cards like GTX 280 or HD 4870 X2 wouldn't even work because they need two connectors each. There are solutions for that but TBH they're not worth it. Here's a quote from a reviewer. He needed two 6-pin PCI-E connectors for an 8800GTX, but two HD 4850 cards need the same thing.
The only thing i dislike is the missing PCI-e power cables. it comes with 1x adapter but this sis still no good if out have a 8800gtx like me where you need 2x 6pin power cables. The adapter supplied only uses 1x4pin molex, but I can''t find another like this anywhere and can only find one using 2 molex connectors. So i have had to use 3 molex power cables for 1 GFX card. Apart from that no issues
It's one of the best PSUs, and with that one you have no worries even with an OCd quad and HD 4850 Crossfire. See, the minimum recommended for those cards is 550W, but an OCd quad is not exactly "minimum" so 550W is cutting it close.
Still, that Gigabyte would do too, if you don't add to many other things (disks, PCI cards, etc.)
What the Corsair 650TX gives you for the extra 4 pounds, compared to this PC Power & Cooling:
3 more A on the 12V circuit - nice, but not really necessary for your setup
The PCI-E connectors have "6+2" instead of "6" pins. You don't care for HD 4850 Crossfire, but it could be handy if 3 years from now you dump those cards and get something that needs an 8-pin connector or two.