I also have a Dimension 4700 which does have Hyperthreading technology. Mine came with a 2.8 gH 520j P4 with H/T tech. The max for that socket 775 board is a 3.8 gH 570j which I upgraded to. You can go to the P4 6xx series processor with a bios flash. I also have 1 gig ram cards in each of my 4 slots. I hve a ati Radeon HD4850 video card. Your board does support pci express x16. Your processor should be fine. I would upgrade your video card & ram though. I can play Crysis , COD2, UT-3, Gears of war, Halo, and Dead Space , all on high settings 1440x900 resolution. The P4 with H/T acts like a dual core processor.
Hope this helps?