Precision 610 II -> III Upgrade ?

I have a Dell Precision 610 (dual PII Xeon 450) with 512mb RAM and would like to upgrade it to dual PIII Xeon. 

Do I need a motherboard swap, or can I just buy PIII processors?

Can I keep the same RAM or do I need to change that also?

If I use PIII-Xeon CPUs faster than 550MHz, will it not work or will it just work at 550MHz?

Thanks.