What is the default power supply in a inspiron 518?

I'm thinking about upgrading my ATI radeon HD 2400 pro video card to a Sapphire Radeon 5670 HD pro video card and it requires a 400W PSU, i just bought this PC and i tried to run Splinter cell conviction and BFBC2 and AvP and all 3 had poor performance with the 2400 card. I saw on amd's site that they had a link to were i could purchase that 5670 for 64.99$ that caught my eye, i have a PCIe x 16 Bus slot + 2 PCIx1 Slots(which isnt needed) but i havent checked out my PSU i was told it was 400W but i want to verfiy that so i dont screw up my machine, if any1 responds to this with a positive answer i will be most greatfull, Tyvm :)