I was speaking figuratively but I did see $4.99 AGP cards on sale here & there though they may or may not have been better than the old mighty GF4.
Is this a fun side project or are you looking to upgrade for main use since ultra cheap deals could be found on the internets that would by far be a wiser move than what you are contemplainting seeing as we are in mid 2009.
Do not be spending money on this system which is just fine as a secondary computer as it is. Key point is that a 2400+, 2500+ or 2600+ mobile Barton (and only those three CPUs), would be the first thing to try to find.
The video card is great for your system, it can outperform many cards that came after it but real upgrade (video card wise) honestly requires a new system so I would keep the good GF4 you have.
I would forget about upgrading to PC3500, it is unlikely you would notice significant improvement jumping a few FSB which your motherboard may not even be capable of, no you need a mobile Barton to raise your frequency, which I assume is 2.3-ish because if you're at 2.4 or 2.5, don't even bother with the CPU.
Your PSU is definitely weak for any major power hungry video cards or maybe even higher overclocks.
To summarize, this system could use a mobile Barton upgrade although frankly, if you're at 2.3 GHz, any gains are likely to be dwarfed by a modern cheap all out upgrade.
P.S. PC3200 is "400" FSB ram by default, if you can't do "400" FSB with a lower multiplier, that means your motherboard can't handle it.