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http://www.motherboards.org/reviews/hardware/1821_2.html

With a TDP of 105W, two of these should be around 200W. Notice that these are 65nm versions, the 55nm should use a bit less. If you figure 200W for the cards, 100W (or more if you have a 125W, 140W, or OC'd CPU) for the CPU, and 50W or so for everthing else, you need around 350W on the 12V rail for this SLI setup.

Here is a review of your psu.

http://www.hardwaresecrets.com/article/OCZ-ModXStream-Pro-500-W-Power-Supply-Review/973/7

They could pull a total of 35A or 420W off the 12V rail. In their test to pull more then the unit is rated for they got a total of 40A out of it before the units OCP would kick in. This is enough power to do what you want.

I'd like to point...
I would have to say no,i think you need more juice.
Most reviews i've read recommend a 450W psu for one card.
Have only found reviews of GX and GTX models = 450-550W psus recommended for single cards.
http://www.guru3d.com/article/geforce--9800-gtx-sli-review-bfg/3
I'm aware your model probably uses less power but i think it's safe to assume a 500W psu isn't going to cut it for two cards.
OCZ FATAL1TY Series 750W
http://www.directcanada.com/products/?sku=13220AC3281&vpn=OCZ750FTY&manufacture=OCZ%20TECHNOLOGY

 
Thanks for the Reply Davcon... I think it actually takes alot less power. I think the 9800 Maxes out at 114 Watts and the GTX Maxes out at like 143 Watts.. Any other Opinions... I only need to use this PSU for around 1 Month and Im switching it into an old PC and getting a new one for my self.
 
A single reference clocked (i.e. 600 MHz core) GeForce 9800 GT graphics card has a Maximum Graphics Card Power of 105 Watts (8.75 Amps from the +12 Volt rails).

A minimum 400 Watt or greater system power supply with a minimum 12 Volt current rating of 26 Amps is recommended for a single card system.

For a two card system in 2-way SLI mode a minimum 500 Watt or greater system power supply with a minimum 12 Volt current rating of 35 Amps is recommended.

Your OCZ Mod XStream Pro OCZ500MXSP is actually able to produce its labelled 36 Amps on its two combined +12 Volt rails. Hardware Secrets was able to draw 40 Amps from the combined +12 Volt rails during its measured maximum power draw of 569.8 Watts from that PSU model.

It is theoretically possible to run your system with two cards in SLI. If you don't over clock the graphics cards you may just be able to get it running.
 
http://www.motherboards.org/reviews/hardware/1821_2.html

With a TDP of 105W, two of these should be around 200W. Notice that these are 65nm versions, the 55nm should use a bit less. If you figure 200W for the cards, 100W (or more if you have a 125W, 140W, or OC'd CPU) for the CPU, and 50W or so for everthing else, you need around 350W on the 12V rail for this SLI setup.

Here is a review of your psu.

http://www.hardwaresecrets.com/article/OCZ-ModXStream-Pro-500-W-Power-Supply-Review/973/7

They could pull a total of 35A or 420W off the 12V rail. In their test to pull more then the unit is rated for they got a total of 40A out of it before the units OCP would kick in. This is enough power to do what you want.

I'd like to point out that I wouldn't get a DX10 card at this time. The fastest card similar to your 9800GT power wise is the 5770. It has GTX260 like performance in the same power envelope as your 9800GT. Two of these in CF would perform much better. I know you're probably looking at total cost however and for the $50-$80 you're probably spending I doubt you'd get anywhere near the same level of performance.
 
Solution
Hey Guys, Thank you alot for your responses. Now I will go out and Buy another 9800gt. And 4745454b I understand that the 5770 Is great with low power consumpsion, But right now, I got a deal on a used but still working 9800gt (I think it Was Asus) for only $40. I may choose to swtich to ATI's cards but right now, I want another 9800 for SLI. I am hoping to upgrade to one of Nvidias new 5 Series with an updated PSU. Anyways, Thank you All for your Replys. Admins may close this thread.
 
I figured you were hoping to add in the second card for cheap. I just wanted to point out that a second card would basically give you a 9800GX2. Going from memory this should be something around a GTX260. Problem is the $100 5770 is also around the same performance level, gives you DX11, and uses the same amount of power as your current single card. Yes its more money, but getting the single card might make more sense. But I also understand that if you don't have $100, you don't have it. (sell your 9800GT for $40 and pony up the extra $20?)

Thread will get closed when you pick a best answer.