To get best value for your owners's dollar (pound) think at least one upgrade in advance.
In other words...
If you upgrade your CPU now will it work on a planned upgrade of the MB? Will RAM you buy now be compatible with the new MB?
Most new vid cards are capable or working in AGP 4X and 8X...so the vid card will last a while.
I plan upgrades along a planned path:
- Full upgrade: Basically the whole thing is toast, time to start over. I try to avoid these as they are expensive.

- Major Upgrade: MB, CPU, RAM. Transfer all the other stuff and get basically a new computer. Still expensive, but far cheaper than a new comp.
- Mid-Life Upgrade: Selected component replacement; CPU, RAM, or Vid Card. By picking the right components in the Major Upgrade I can keep a valid upgrade path open to keep my old comp workable at very little money.
I'm in the middle of a Mid-Life crisis...errr...upgrade right now.

New vid card (ASUS 5700) and replaced RAM DDR 2100 out and DDR 2700 in. The RAM upgrade was unplanned...my old 512 stick was glitched and I asked what the price difference was between the free warranty replacement and the better stuff.

Cost me $35.00 CDN for the upgrade.
You also have to consider how much of the work your owner is capable of/comfortable with. I do all hardware work on my comp myself...frightening how much that has saved me.
I started with a PIII 500 (Amiga Owner previously). Built by a local comp house.
Upgrade 1: More RAM
Upgrade 2: New MB and Case/Power supply
Upgrade 3: New HD
Upgrade 4: New CPU (800 Celeron)
Upgrade 5: Major- New CPU, MB, Case and RAM (1.7 GHz PIV)
Upgrade 6: New Vid Card
Upgrade 7: Added CD Burner and Scanner
Upgrade 8: New RAM and New Vid Card (Current)
Total cost of all upgrades to date, doing the work myself, is not up to the original purchase cost of that PIII 500.
