what kind of motherboard do i have to get?

i was trying to install an ati radeon 9550 pro in my old 2350 dimension (dell)
but i couldnt because it doesnt have any pci or agp slots , so now i think i have to get another motherboard but i dont know what kind

your dimension dell is an oem build and therefore you cannot buy standard parts to replace the old ones. it isn't guaranteed, but most dell cases and parts will not accomodate a standatd (micro) atx board.

thanks< you know how can i find out if it would fit ? is there like a webpage or something ?

the only way you can find out is by measuring out the box yourself/trying to fit an atx board. some people could tell by looking.

also there are mounting style issues and PSU issues (IE I've heard PSUs from dell are wired differently)

that is correct. really the only parts of oem builds that generally are compatible with other pcs are the drives, cpu, and most expansion cards.