I know this is a homebuild forum but, I just gotta ask something. I got a beautiful HP Media Center PC from Best Buy a few years ago and I love it like crazy. Last Christmas, I was going to upgrade my video card from a Radeon X300 to a XFX Geforce 7600 GT. I was going to check with HP about my PSU and found out that my PSU is a 300W!!!!! I'm running a Pentium D 2.8Ghz, 2 Optical Drives, 250GB SATA HDD, and other bells and whistles on only 300W. They cheated me. I was bummed and heartbroken ( until I got my DS Lite). So know I will have to get a new PSU and then get a card. I am planning to build my own down the line (after this and other incidents, I am sick of retail computers other than laptops). So I wonder if I should even bother. Yet, I still love my HP and I want to extend its life a few more years. What should I do?
I picked up a compaq last year with similar specs (but with AMD 3800). The best thing you can do for these is to upgrade the video card. I bought a 7600gt and a SeaSonic PSU together and it was seriously like night and day. I can play FEAR with most settings at MAX and 1440x900 resolution now with a 45fps average (not stellar or record breaking, but playable). A full size atx PSU can fit in these cases, but make sure you have a place to stash the extra wires. My temps went down 5c once i reworked my wiring job. I dunno about ur pentium D system, but i was able to upgrade to an AMD 4600 and hopefully make my pc useful for another year or two.
I'd say buy a decent psu now, but don't plan on it being useful for your next build unless its real soon. This is because specs and connectors keep changing as new power requirements and add-ins become available. Then I'd buy a cheap video card like the 7600GT or the 8600GT until you build your new system.
You buy a good replacement PSU. Go to a better company like P&PC and they make special PSU for Dell and such. They will know what you need to get and advise you all the way. It won't be a cheapo but the machine should last a couple more years as a media center. I love the 7600 series as you want to use one. My DVR will have the same series.