The fastest CPUs supported by that motherboard would be either the Athlon 64 x2 56800+ or 6000+.
You'll want to make sure you are on BIOS 1.1.11:http://www.dell.com/support/drivers/us/en/19/DriverDetails/Product/dimension-e521?driverId=R164018&osCode=ww1&fileId=2731117833
You'll also need to make sure you have the heatpipe cooler and not the aluminum block version.
For the GPU, the power supply does not have a 6 pin for additional power. Something like a GTX 650 or a Radeon 7750 would be best among brand-new cards. The BTX design means you need to have a single slot video card.