Power supply and case help

Hi. I'm having problems with my power supply. I'm deciding between a few models and while I'm at it deciding if I should upgrade my case too. My current case and PSU is here:

Antec SLK1650B

I'm deciding between the following PSUs:

Antec NeoHE 500W - $90 shipped
FSP AX 450 450W - $70 shipped
Hiper Type-R 480W - $70 shipped

I'm considering the following case:

Antec P180 Black

I don't overclock so I don't think I need the new case, and I like the small form factor of my current Antec case, but since it's on sale I wanted to check that I'm not missing out on anything I'll regret later. I have the following components that are more than adequate for my current needs.

2 WD SATA hard drives
1 WD IDE hard drive
1 DVD RW drive
1 DVD Rom drive
1 Abit NF7-S V2 mobo
1 AMD Athlon XP 2400+
1 Asus A9550GE
3 Crucial 256MB DDR266

Thanks!

Hi. I just received my new PSU an FSP AX450-PN. I hooked it up and I get an error on my -5V. It's reading 3.55V, way way above the alarm threshold. Is this normal? Is the PSU faulty or is it possible that it's incompatible with my mobo? I have an Abit NF7-S v2 mobo. Thanks.