So sorry -- I was NOT clear. I'd suggest sticking with the 32 bit you have already loaded... use it, it's there. If down the road you find issue then consider using 64-bit. Use what you have first (32bit OS), add the video card so you have HDMI, tuner.... wait a minute... you never said you need HDMI for anything!
What TV and how do you plan to connect to it? HDMI? If you're using extenders for the PJ then you may not even need a new video card... check your Device Manager and see what Video card your system has integrated - it may be sufficient, TRY IT! Load the latest drivers as well before you give up although If you need HDMI or something more powerful then spend a quick $20~30 on a HD4550, HD5450, or HD6450 and you're there!
Once the PC is displaying as you need on the TV, load/play a DVD and check out the TaskManager's CPU usage. This should be an easy test and should pass without much trouble seeing as DVD is only 720x480... without having a HD tuner you could download some sample mpeg-2 files (.ts or something) to stress your CPU/GPU even before you buy the CableCard device. These are baby steps toward your final system. You don't have to do them if you feel comfortable buying the video card, CableCard device and such as one step.