If you put in a video card it will supersede your onboard graphics.
As for the 750w recommendation, many manufactures will overestimate the wattage because there are a fair amount of PSU makers that fudge the numbers to get higher wattage ratings. A 4870 will use about 130w and is limited by it's power input to 225w (75w from the pci-e and 2x 75w from the 6-pin connectors). That 750w is more than enough for 2 4870s in you rig.
Doing a load power profile.
70w - Phenom II X4 940
20w - Gigabyte GA-MA790GP-UD4H AMD
130w - Diamond HD4870 1Gb
8w - PNY Memory XLR8 performance edition DDR2 1066 MHz 2.1 V 4Gb
14w - Seagate 1TB 3.5" Sata 7200 RPM H
15w - Creative SB X-FI Xtreme Audio P (54.99$)
20w - dvd, fans, bling
~277w total
So you have a fair amount of play in your system.