The HD3850 is the 2nd best AGP card available...the HD4670 is better. But your system is old & outdated. If you were running an Athlon64 AGP system, it may make sense to go with one of these cards, but since you're running an Athlon XP, you should set your sights a little lower. And it's obvious you don't know much about power supplies because you're basing you choices on wattage & price rather than amperage.
Look at the specs to the 400W Corsair:
+3.3v at 20A
+5v at 20A
+12v at 30A
The amperages are well distributed. Very few components rely on the +3.3v & +5v rails so 20A on each is plenty. The +12v rail is the "mother of all rails" & having a single +12v rail allows all components that need it to "feed" from the full 30A.
Now look at the 500W Coolmax specs:
+3.3v at 30A
+5v at 45A
+12v1 at 18A
+12v2 at 16A
With a split +12v rail, one rail is used solely by the CPU & can not be accessed by the other components, so all must "feed" from the other +12v rail.
Now lets do some math. The AXP 2800+ is rated at approx 68W & gets all it's power from the +12v. So if you know the formula (watts = volts x amps), you can figure the amps needed by the CPU. In your case, it's approx 5.7A (68/12). If the CPU uses the +12v2 rail, approx 10A (16a -5.7A) are "wasted" because they unuseable by the rest of the hardware.
Now, look at the ratings of the other rails. Since 20A is reasonable rating for the +3.3v & +5v, anything over that amount is wasted.
3.3v x 10A (30A-20A) = 33W (waste)
5v x 25A (45A - 20A) = 125W (waste)
33W + 125W = 158W (total waste)
500W (PSU rating) - 158W (total waste) = 342W (useable)
As far as useable power is concerned, the Coolmax 500W is closer to being a 340W unit than a 500W unit. And if you take into account the 10A trapped in the dedicated CPU rail, the useable power drops to about 220W.
You really need to rethink what you're doing.