for what you have listed there a 500w earthwatts PSU would be fine, only problem you have not listed what graphics card you are looking at,
i mention this since if you are looking at the new gen cards (especially the gtx260/280) it appears that you need to have your own Nuclear power station to run them (i beleive it is around 240w a card) in which case you will probably need to up your PSU to a 650w or above with a 8 pin AND 6 pin connectors, if you are looking at an 8800GTS or lower then the 500w is fine
well the PSU should be fine then, but it might be worth while to look into the crystal ball to see if you might upgrade your graphics card in the future as a 500w will not run the next gen cards let alone the cards after that.
but then if you are like me and get a system and then run it for 2-3 years without upgrading (unless absolutly nessesary) and bank all the money in a savings account that you would have upgraded with, then build a whole new system from the latest parts then you dont need to worry. this ofc is a personal choice.
next thing thou is your motherboard choice, i have personally never heard of DFI, however i do live in the UK so they might not have branched here yet, i personally would go for a gigabyte p35 DS3L as i know they are very good boards
Next HDD i beleive the WD caviar are a little bit loud, the flavour of the month at the moment in the UK is the samsung spinpoint F1 HDD's as reviews have shown them to be nearly as fast as the WD raptors yet a lot quieter and a lot cheaper