Joking aside, I looked up the power consumption of running a crossfire 4890 system with all of my LED fans, CPU/RAM voltage, HDDs and USB peripherals. It rendered a PSU of around 620W. I'm currently running a 700W PSU, but I'm still a bit iffy - even though the Sapphire site says a CF system needs a 600W. Would my PSU be able to dish out what a second 4890 needs?

What kind of PSU (Brand) are you using? The 12V and 5V rails are important to check for amperaged, A good quality PSU with a decent amount of amperage on a dual or quad rail 12V and good amount on 5V is important. I would suggest checking out other system setups online and comparing your PSU to theirs, including all fans, opticals, leds etc...and making sure your psu falls in the average range of what you see in others.

Cheers,
Brent

Right on Brent.

Gilly, Also keep in mind an overclocked card is going to want more juice. With what you are running I usually do a little overkill and head for the 7-800 range, just for piece of mind. I run an Ultra Modular 850 in my main rig. Good to have the power when ya wanna start overclocking things and I would always go brand name. You also wont have to worry if you need to add a few things later. ( if u have room left )

It doesn't look like the greatest configuration, but it should be ample enough given your system.

Thanks for the input guys - and what I did was compare my rig to what other have, but they usually have a 1000W or so with too many other things juicing the PSU to do a proper comparison. However, I did come across (as a majority) that 800/850 seemed to be the norm, but some are running 750. Of course, like Hali said, OC'd cards take up a bit more power.
Unfortunately for me, money is a bit tight at the moment, and building up to another 4890/CrossFire MB is gonna hit my wallet where it hurts. What I'm trying to do right now is see whether or not I'd need to spend the extra 150-200$ on a new PSU and add that to my total spendings.

You should be fine with your current PSU.

Use this tool to calculate what wattage you'll need to run your configuration.

http://extreme.outervision.com/psucalculatorlite.jsp

EDIT: Also make sure you purchase a PSU with Active PFC.

I've recently turned myself into a kijiji-tard.

But yielded some amazing deals on hardware on purchases made wrong and people who needed the money after buying something brand new.

for example, in about 30 mins I am having an AMD Athlon X2 7750BE ( 1 month old ) dropped off for 50 bucks. Cheapest local i can find new - 105 bucks

I picked up several raptors with 3+ yrs warranty for 50 bucks a pop. the list goes on. Keep ya eyes open, and something will come your way. Be sure you test things of course before handin over the doh - or you will just end up saying 'doh'

Or go pick them up their place of residence and do up a small reciept.