What is the best cpu available for a stock dell dim 2350

Alright, the best I can tell from this spec sheet: http://support.dell.com/support/edocs/systems/dim2350/specs.htm#1106961
The fastest proc you can do is an old Pentium 4 Socket 478 with an FSB of 400 and a Clock speed of 2.5GHz.
According to Wiki here: http://en.wikipedia.org/wiki/List_of_Intel_Pentium_4_microprocessors
Your best bet is part # RK80532PC060512 Which is what was intended to be the fastest proc for this mobo.
However, I believe you could get away with the fastest Northwood processor which was a FSB 533 and 3GHz. But the ability for this to work is dependent on your BIOS revision. Good luck finding one.

All that said, there are other upgrades to consider if you are looking to upgrade your computer. Best bet is a new HDD. Any modern HDD is going to be worlds faster than the one that came with the PC, and this will be the largest choke point of the system. Just be sure to get an ATA100/133, not an SATA.
Also the fastest PCI video card available would add a large boost. My choice would be this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814500221 but you would need a 300W power supply instead of your stock 200W. For your stock power supply then you should do this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814133364

Still, your computer is painfully slow. It is really time to get a new base system if you can afford one. Even a simple duel core would be a huge advancement to this. For $300 you could get an AMD apu (processor+graphics), mobo, 4GB ram, 160GB HDD, cheap case, 250W power supply, and a cheap optical drive. Obviously a larger budget would give you better hardware. Hardly a high end system, but it would beat mine on many tasks XD