If you decide to go with an AMD chip (which most people do), get a Socket 'A' mobo, something like the K7S5A this will take a duron, or XP. Some boards only go up to say a 2.4 XP, but bios updates will allow them to go higher. If you go for a lower spec board with SD ram, you will be able to use your current ram in it. If you go for a board with DDR, you will need to replace it. Some boards have SD and DDR sockets, in which case you can replace your SD for DDR when you get some more money.
As for the deals, I am in the UK, so can't really comment. I get my stuff from EBuyer which I believe is also based in the US.
The board you have BTW is a Slot 'A', the cpu is a sideways mount with fan built in. The newer boards are socket which are flat mount, you will need to get a HSF as they do not normally come with the chip, unless you get an AMD retail kit (cpu and hsf), or a complete package (mobo, cpu, and hsf).
As your current spec is ATX, all you will need to do is swap the mobo. When you switch on, Windows will probably detect new hardware from the board and will attempt to install it, in which case I would recommend backing up important software before doing this incase it go's tit's up
HTH...