what hardware do i need to max out my dell optiplex gx280?

Solution
That's a Pentium 4 system, unless your into retro-gaming then there's not a whole lot you're going to do with it.

What were you hoping for?
I agree with popatim.

https://www.dell.com/community/Desktops-General-Read-Only/Optiplex-gx280-processor-upgrade/td-p/3615041 shows possible CPU upgrades. And https://www.dell.com/community/Desktops-General-Read-Only/Optiplex-GX280-Maximum-memory-capacity/td-p/1965796 shows RAM upgrades. There isn't any upgrade that'll make an appreciable difference.

If you want to get technical you can download https://www.ccleaner.com/speccy. Paste the summary report here or post a link to a screenshot. You MIGHT be able to upgrade your GFX card. One possible issue there is that I think that may be a SFF aka a small form factor which means it's very short so it needs special low profile GFX cards. Another issue is that if you can upgrade the GFX card the CPU and low RAM amounts will negate the upgrade.

In short, you can't.
 
Oh Prescott CPU, I remember having XPS Gen3 with similar CPU, I think I could play Far Cry 1 and maybe FC 2, Soldier of Fortune, first Wolfenstein, first Doom, first Half Life and things like this, I think I had 8800GT there as well.

Right now it is tolerable only for Internet surfing with some very light browsers like Bravo or Vivaldi, no movie streaming.
I think this is it.