With regards to the processor, at the time I bought my 420 in Dec 07, the Quad Extreme QX6850 WAS the fastes processor available for it.
I have no idea about any processors that have come out since then. Sorry.
Keep in mind that when you get into the territory of 'extreme' processors from Intel, they fetch a premimum price. In my case the QX6850 was a $1,100.00 option. Going higher might not be cheap.
Supported Processors on the 420
DX483 Conroe Kit, E4400, 2.00G, 2MB, 800FSB, L2
DX484 Conroe Kit, E6420, 2.13G, 4MB, 1066FSB, B2
DX485 Conroe Kit, E6320, 1.86G, 4MB, 1066FSB, B2
HU587 Conroe Kit, E4300, 1.80G, 2MB, 800FSB, L2
MP160 Conroe Kit, E6550, 2.33G, 4MB, 1333FSB
MP162 Conroe Kit, E6750, 2.66G, 4MB, 1333FSB, G0
MP628 Conroe Kit, E6850, 3.00G, 4MB, 1333FSB, G0
MP626 Kentsfield Quad Core Kit, Q6600, 2.40G, 8MB, 1066FSB, G0
MP627 Kentsfield Quad Core Kit, Q6700, 2.66G, 8MB, 1066FSB, G0
UP673 Kentsfield Quad Core Kit, QX6700, 2.66G, 8MB, 1066FSB, B3
RX656 Kentsfield Quad Core Kit, QX6800, 2.93G, 8MB, 1066FSB, G0
PW016 Kentsfield Quad Core Kit, QX6850, 3.00G, 8MB, 1333FSB, G0
WM543 Conroe XE Kit, X6800, 2.93G, 4MB, 1066FSB, B2
see here for more info XPS 420 FAQ
Tim thank you for all your help and you have told me what I needed to know. Just wanted to get the info correct now so that when one comes along (Processor) I can pick it up. Looks like the 300 quad is the way to go. And for the money hopefullu in another 9 or so months they will be a little cheaper.
Thanks for all your help and have a safe and happy 4th of July.