We need to know what system components the PSU will be powering. The Seasonic S12 series is great and very quiet. If you have the money, want quality/quiet and it supplies sufficient power for your rig, then That is the route I would go. Max amps on the combined 12V rails is 28A - you need to be sure that amperage is sufficient to power your system components.
An interesting note is that the S12 has the same max power (thus you can calculate the amperage rating) for the combined 12V rails as the AX450-PN. And that is with FSP rating 18A on each 12V rail and the S12 rating 14A and 15A. At least FSP has the honesty to put max combined power on their label. Some mfrs don't do that and some do it to intentionally mislead consumers - a sad state of affairs. One more thing, the difference between the AX450-PN and the AX450-PNF is Power Factor Correction - a PSU requirement in the EU.
Well mine is a pretty old system, and i just bought a new 20" monitor with DVI support, but my currrent graphics card does not have DVI, so in order to get a decent graphics card with DVI, i'd need a new PSU.
Current PSU = Model - FSP250-60GTA (MDN)
Max output 250w
If this is to be able to upgrade the GPU, then what GPU will you be using? Still need to know the rest of the specs. For a cheap upgrade I would go with the FSP - assuming it has the juice to power your system. I use them a lot and they are dependable, inexpensive little PSUs.
Well, I have that FSP PSU and aside from RMA'ing once, it has been good. It is EXTREMELY quiet. I never hear it. The cabling is flexible, so it is easy to hide. The PSU has about 25 amps available to the 12v rails, so 8800GTS is the max I would go on it.
Intel P4, 2500 Mhz (25x100) (northwood)
512 MB (pc2100 ddr sdram)
mobo is a Msi ms-6533
Ati radeon 7500 (going to upgrade, since i need DVI)C
Current PSU - Model - FSP250-60GTA (MDN)
External 250gb SATA hard drive
Wireless Laser mouse