Generally, the speed is limited by the Intel chipset.   However, as Dell many times does custom implementations even if the chipset has a higher speed it can be limited by the Dell implementation.  Thus we can't fully answer your question.

An example of the Dell custom, on the Dimension E510 model, Intel listed compatible CPU's but because of the Dell custom, not all the Intel listed CPU's were compatible and worked in the Dell.

Faster ram does not make a system faster.

pc2-3200 ddr2-400 fsb 200mhz
pc2-4200 ddr2-533 fsb 266mhz
pc2-5300 ddr2-667 fsb 333mhz
pc2-6400 ddr2-800 fsb 400mhz

Cpus with 400mhz requirement usually do not work with the slower ram.

Some systems with 333mhz fsb do not like 400mhz ram.

There is no one size fits all for all dell models.