HP DC7100 (sff) will these upgrades work?

If you haven't bought anything yet, I would sell your setup on ebay and start over, unless it was free. DDR cost twice as much as ddr2. Your power supply is a special size, and the replacements are only available up to about 300 watts. Your pc wasn't designed for gaming, just for internet surfing and word processing. If you don't have the ability to change the motherboard, find a vender who can ship you a barebones setup with case, board, cpu, heatsink and memory installed, so all you have to do is transfer the optical and hardrive.
Otherwise, I would get a friend who has more experience and get a new case and power supply with about 450-500 watts. Antec, seasonic, pc power and cooling, corsair, and enermax are my favorites, followed by fsp fortron and ocz. Pricewatch has some amd barebones setups starting at about $110 shipped. Add 2 gigs of ddr2 for $20-25, your old hardrive and optical drive, and your video card, and you'll have a better gaming pc.