The 350 watt power supply will easily run most systems today. Since the PC Power & Cooling PSU's are so high quality, they provide really clean power to the system and I wouldn't be surprised if a 350 watt couldn't actually push out over 400 watts at peak usage. I wouldn't waste my hard earned money on a 425 watt PSU knowing I didn't need that much power.
That PSU tester is a nice thing to have, but how often do you think you'll use it? I put a couple of computers together a month and I don't own one of those. I've seen the Antec PSU testers at CompUSA, but never thought I needed one.
Cricket