Power needed for Radeon HD 3850?

i realize this topic has been covered several times on this forum, but i just wanted to make sure it was a possibility with my specific system before buying the card.

I have an old Intel D875PBZ mother board thats AGP only, with an also aging Radeon 9800 pro on it. I was interested in upgrading to a more powerful AGP card and came across the HD 3650 and 3850, and am currently leaning more towards the 3850. the only problem is my psu is 420 watts with 18A on the 12V rail, and the card reccomends 450 watts. is this enough?

my set up is:
Intel D875PBZ Motherboard
two P4 3.0 Ghz Processors (this computer was a gift and didn't even realize it came with two in it, i don't know how that will effect the power draw)
4GB PC3200 DDR RAM (4 x 1GB)
Seagate Barracuda 5400RPM 80 gig IDE drive
Seagate Barracude 7200.11 (7200 RPM) 500 gig SATA drive
Sound Blaster Audigy Value PCI card
420 Watt 12V 18A PSU

the 3650 only requires a 400 watt, which i have, but the perks of the 3850 seem worth the extra money. I realize this all old components and i'd be better off saving up to build a new computer, but i don't really have that in me, so a new video card to stall its death a couple more years seems more practical for me.

here are the two cards i'm looking at:

3650
http://www.newegg.com/Product/Prod [...] 6814102814
3850
http://www.newegg.com/Product/Prod [...] 6814102730

^ agree with checkout, even a dated 420w should be fine for the 63watt HD3850 AGP