Personally I think your 300 will be fine with that config. Make sure you have enough 12v output... the p4 has the extra pull for 12v.
The Antec power supply should be decent quality (not fake over-rated), so I wouldn't worry too much.
My kick-ass system I am building out of Anandtech threads:
-- 4u rack mount case - black (compugeeks) / Dynapower 350AT power supply (mwave.com)
-- P4 1.6A @ 140Mhz FSB = 2243Mhz (Haven't tried higher yet)
-- Asus P4S133 motherboard (mwave.com)
-- Plextor/Iomega 12x10x32x CD-RW (anyone remember CompUSA January 1 / 2001?)
-- Digital research 16x DVD-ROM (hey, I needed it to install Windows XP in hurry, paid a sucky $79 at Best Buy - yes, my Windows XP was a DVD disc!)
-- two Ethernet cards - LinkSys LNE100TX Version 5.0 (Best Buy)
-- Matrox G550 Dual Head 32MB AGP (mwave.com - man, tempted by the new dual-head Geeforce4 - but I don't really need 3D)
-- Planar 17.4" LCD, 1280x1024 (Dell.com)
-- Slyvania 17.0" LCD, 1280x1024 (Costco)
-- 80GB Maxtor 7200RPM in a removable 5.25" pull-out tray (mwave.com tray)
-- Promise Ultra133TX2 (reflashed firmware on Maxtor controller that came with 80GB drive) PCI. I have the hard drive on th Promise controller, CDROM + DVD drives on the on-board controllers.
-- Antec PCI to PCMCIA adapter, running my Orinoco 802.11b wireless (this machine is a 3-way router, two ethernet + wireless)
-- I added a 2nd sound board, just for kicks. PCI 5.1 channel one I got at mwave.com
Missing: RAM. Right now I'm running a single 512MB PC2100 mwave.com module. It is going back, not reliable.
-- Crucial 3 x 512MB PC2100 modules = 1.5GB, on order.
-- I also don't have any speakers, yes, two sound cards and no speakers
EVERY SINGLE slot is full on my Asus P4S333 - working great!
I was going to return the Costco 17" LCD and just use a 17" LCD + 15" Sony monitor I have (save $$), but I like having the dual 17" LCD so much that I may end up keeping it. The pivot software that came with the Planar allows the Matrox do run one at 1280x1024 and the other at 1024x1280 - nice.
This is my main software develop machine. I figured if I was going to run $2,700 worth of software on it (Microsoft MSDN Universal Subscription) I would at least do it in style
Thanks fellow Anandtech members for all the savings. I'm broke now