What can I do to optimize my HD2600 Pro and get the best out of it.

I have a fan system so I cant overclock to much if at all sooo what are my options?

Thanks

P4 2.8 Ghz
2gb DDR400
Ati Radeon HD2600 Pro
P4SCT+II MotherBoard

This tool will allow you to OC the video card's core and memory--
but be careful. You really don't need to push things too hard.
The tool will allow you to find the maximum for your setup easily
then you can save those to a profile-- so you can run normally-- or in game-mode with one click.

ATITool - Overclocking utility for ATI and NVIDIA cards

Will it be worth it overclock my card and processor for my system? say, could i bump up one graphics level on crysis?

Card-- probaby-- try it.
Processor ? some of the older P4's like mine (and yours) cannot be overclocked... locked multipliers.
With ATI tool it looks for the highest settings -- that are safe FOR you.
Crysis-- in particular? no idea.
ATI tool can help you get the most (if you're careful) out of your card.

will i be ok with a fanned system, i have 1 80mm in and 3 80mms out, the card has a big copper heatsink on the back and a exhaust fan on the front, i have a pci slot fan below the gfx card the sucks in the hot exhaust air from card and blow it out the back. am i good for a minor oc?

It sounds so-- but one clue-- take it up slowly.
OC is a matter of moving things in small increments -- take it up a little, let it stablize, take it up a little more- let it stablize... etc.
When you start seeing little, quick flashes on your monitor STOP... and back off just a little. Then leave it alone.