a good 24" HD monitor is goin to run you at least 300 bucks, I paid 500 for mine about 6 months ago. the monitor you have will do now good with new hardware.
if you decide to upgrade your monitor, think about nvidia 9800 gtx oc video, they run about 200 bucks, so 2 is around 400 dollars. the intel core 2 duo is a work horse and cheaper for your budget. 4 gb of ddr 2 ram will do just fine, you can up it to 8 later on.
look at newegg.com parts and prices, I am just throwing ideas to stay in budget.
