isaacw

Base upon your budget (HK$700?) and you are a casual gamer, I would susgest to wait a little longer untill US Black Friday sales is on (Nov 25), I suppose you can order certain on sale nVidia GTX460 1GB DDR5 OR ATI 6850 1GB DDR5 cards at around US$90 after rebate which shall fit your budget and also good for running recent games at 1680x resolutions at medium settings. However a nice 80 Plus certified 500W PSU is highly recommanded for these cards.

If you must get a bang for the bucks video card right away (save the cash for a better card later), consider XFX GeForce GT 240 550MHZ 512MB 2.0GHZ which is an overkill for just watching HD or blueRay movies on an i7 processor. This card is on sales at CAN$20 after rebate. Ultra low power consumption for HTPC too.
Check this URL:
http://www.ncix.com/products/ind ... FX&promoid=1366

For your questions on whether DDR5 比DD3好, you must also need to know the memory bandwidth such as 128bit, 256 bit or even higher on the specification of the card.