The Intel 82915/GV/910GI Express Chipset Family says that it has 128 MB of video memory, I am not sure it this is shared or dedicated. It is much better if it is dedicated.
I have a Pentium (Let me see , (I have been around FTA too long )) P4 3.0 GHz, 1GB RAM, Radeon 9700 128 MB graphics card and I can view most FTA HD, occasionally the 1080i 4:2:2 37 Mbps feeds give me problems, but I can recored them and then play them back later with out any freezing.
If anything, you may be low on memory. 512MB is not a lot, especially if your video uses shared memory. In general, Vista likes more memory than XP as well. With memory being so low priced, I would recommend upgrading to 2 GIG of memory even for everyday computer use. A computer low on memory is very sluggish.
Check here for a fair price (in Canada). Just check first for what type of memory your motherboard uses to be sure you get the correct type.
Motherboard that support dual channel memory, will give you a performance boast if you install 2 memory modules of the same type (or a packaged dual channel kit). You would have to remove the 512mb installed for it to run in dual channel mode.