I've got a Dell Dimension E521 currently running AMD Athlon 64 X2 4400+ and would like to know what processor is the maximum I can install with the standard Dell heatsink and fans?
The max processor upgrade is the Athlon 64 X2 6000+ 89W 3.0Ghz Windsor. It's a modest improvement to what you have, but unfortunately you're a little late in thinking about upgrading as they don't make that processor anymore. The Windsors were replaced by the Brisbanes and for the most part they do not work in E521. You can try some place like eBay, but they're very hard to find. You may find a 125w version of the 6000+ Windsor, but not advisable since it runs to hot for your heatsink/fan.
Vostro 460 – Intel Core i7 2600 @ 3.4Ghz - 16GB DDR3 @ 1333MHz –Kingston 240GB SSD – Seagate 320GB HDD – ASUS GeForceGTX 550 Ti 1GB DDR5 – 16X DVD-RW – 16X DVD-ROM - Antec 550w PSU - Windows 7 Home Premium X64 SP1- Dell E2211H 22"Flat Panel Monitor