9800 gt 12V spec

Depends on the brand and model of the PSU, the specs should state a maximum combined wattage of the 12V rails, example: http://xtreview.com/images/psu-corsair_02.gif , here you can see that for example the Corsair HX520W has three 12V rails of 18A, but the maximum combined wattage is not 18A*3*12V=648W but only 480W, in contrast the rails do add up with this OCZ 400W http://www.ocztechnology.com/drive [...] tChart.jpg if the specs don't state a maximum combined wattage it probably means you can just add the rails, but not always, so I'd ask the vendor/manufacturer about it first.

By the way, Watt(W) = Amperage(A) x Voltage(V)

I don't know: the max combined wattage isn't specified on those psu's, so you either have to e-mail the manufacturer for that or just get something else, like a Corsair/Antec/OCZ.

It also depends on your other components, how many hard drive's do you have, what kind of CPU, etc... If you have 1 hard drive and a typical 65W (core 2 duo or athlon X2) or 95W (core 2 quad) CPU you need 22A or 23A on the 12V rails, if you have something bigger like a 125W (Phenom X4) or 130W (core i7) CPU you need even more power.

Your best bet would be an OCZ Stealthstream 400W, or a Corsair CX400W, those can power your 9800GT + an overclocked quadcore and 1 or 2 hard drives.

You'll need 20amps to run the system (you can leave the dvd drive connected), but make sure you have an "80plus" certified PSU, otherwise the PSU may not deliver on its rated specs.

Your system with a standard 9800GT needs 20A, if it has the "Green" 9800GT you need 18A.

But please make sure your card is a green edition (TDP = 75W) because I couldn't find the TDP of that particular card on google.

now i have a 450w mercury smps.it has 18A in 12V rail .is that enough to run the system and the 9800 gt card

Huh ?
The efficiency rating has nothing to do with wether or not a unit can handle high loads.

I personally wouldn't trust that Mercury to power anything.

Gulli - There's a minor problem with the ratings. The tests are for power supply efficiency and yes they do test the psu at it's full load as shown on the psu's label. From what I read in separate technical reviews there were a few bronze rated psu's that either failed or were below their efficiency rating when the cpu and gpu in a system were at their full loads.

I checked the institute's test protocol. The problem is that the protocol does not involve using an Intel Core i7 with a high end video card. In fact, there is no pc used at all. It is the same problem we have with technical reviews. Results vary because different criteria and different test platforms are used.