Upgrading Graphics and Power

The PSU has the voltage, but I don't think it has the connectors. It looks like the only PCI-e power cable you've got is an 4-pin molex to 6-pin PCI-e adaptor. If you want to continue using the same PSU, you'll need to find a 4-pin molex to 8-pin PCI-e adaptor cable. I'd give you a link to an example, but can't seem to really find one.  
The video card you chose needs both of those cables to be able to work.

But this may all be irrelevant because that is a powerful video card and at stock speeds, your current processor won't be able to take full advantage of it. I'd honestly recommend upgrading your whole PC at some point rather than just the video card.

Hope I've been helpful.

Thanks a lot for your informative reply.
If I were to upgrade to the 4870 X2 I wouldn't hesitate to OC to around 3GHz, which is still stable on air. Do you think this would still create a bottleneck?
If not, then I will have to try and find a solution such as the adaptor you mentioned.

If you're going to overclock after you get the video card, then you really should replace your PSU too. It's not so much a problem with the wattage, and I may just be paranoid, but I don't trust your PSU manufacturer. If the PSU fails, it can take everything else out with it, and overclocking will add additional stress. Assuming your PSU's also a few years old (going to guess around 5 years) then it is probably due to fail sometime anyway. I don't want to scare you, but PSU's scare ME since they have the ability to cause a lot of damage.

Again, you may find that the risks aren't significant enough to warrant buying another PSU, but if you are going to look at getting a new one, I recommend the Corsair 750TX. They also come in 650 and 850W flavors, but 750 is probably the one for you. These are very good PSU's, and they also have all the cables you'll need, even for SLI/Crossfire.

Anyway, overclocking to 3GHz probably won't bottleneck nearly as much, and the Q6600 can overclock like a beast... so combine that with new card, and it may take you're machine from an average gaming machine to a damn respectable machine.

These are just my personal opinions, and I hope you'll get some more people's advice too.

+1 for kufan's suggestion of overclock and PSU change...
Before I go ahead and suggest any options, @ what resolution do you play @?
If you dont go over 1920X1200, then save some money and go with the HD 4890 1GB...
That graphics card can easily handle upto that resolution easy...And 4870X2 would consume more power and produce more heat...
And ATI is set to launch newer DirectX 11 cards during October or by latest end-of-year...So for now invest in the cheaper HD 4890, save cash and in about a year upgrade to the newer cards...

I definitely agree with Kufan. If your PSU is so old it doesn't even have a PCIE power connector, there's no way I'd trust it to power a 4870X2. Choose a replacement from a quality brand like Antec, Corsair, PC Power & Cooling, Seasonic, or Enermax. You won't be Crossfiring on that mobo, so 650W would be plenty.
Using the PSU calculator at http://www.extreme.outervision.com/PSUEngine I get 521W (based on some assumptions about drives and fans) with only 10% capacitor aging (e.g. for a new PSU), but 615W with 30% capacitor aging (for your old PSU), which will almost CERTAINLY smoke the old one. Its 700W label is a) probably overrated, and b) likely too weighted to +5V; a more modern PSU has more wattage on +12V where it is needed.

Thanks alot for your replies again.
I am gaming on a 22" at 1680x1050, so I will look into your suggestion with the HD 4890.
Also, the power supply is actually less then 6 months old, due to the last one stopped giving power on half the wires.
Despite this, I do agree I would be worried about losing something if the power supply rating was too high, so I will also look into this.

I've just looked over the options, and have come up with a few new questions.
As the HD 4890 would take less draw than a 4870 X2, and taking into account the PSU is actually relatively new, would I be able to keep the power supply? And, if so, does it have the same connectors as the 4870 X2 (I'm assuming not due to the X2 being dual GPU).

The HD 4890 requires two 6-pin PCIe power connectors. This is much better, since 8-pin connectors seem to be a pretty rare breed. Wherever you intend on buying it from, make sure that it includes at least 1 6-pin PCIe converter cable, as you'll need it.

I'm pulling this off of a product from newegg.com:
Package Contents:
HD 4890 card
User's manual
Drive CD
2 x Power cable
HDTV cable
DVI to VGA/D-sub adapter
DVI to HDMI adapter
S-Video to composite adapter
CrossFire bridge

Those power cables are probably the converter cables you need.

To attach this 2nd converter cable, you need two more 4-pin molex connectors (that you aren't using for anything else) coming out of your power supply. Your PSU's specifications say that you've only got 6? molex connectors coming out of the PSU. I can assume that you're using 2 for the video card now, and one for the your CD drive, and maybe 1 or 2 for your case fans. If your HDD is using one too, there's no way you'll have enough free molex connectors for the new convertor cable anyway.

They make molex splitter cables that take one connector, and turn it into 2 but I really have to recommend against this approach for a number of reasons.

I hope I haven't just confused you. Let me know what the situation looks like. I still recommend a new PSU, though it is a bummer that the thing's only 6 months old.