A few things you can do. One is go to dell's website and plug in your service tag info into the website and you'll get a list of your components when the system shipped and you can opt to have dells website query your rig as is by clicking current configuration. Or use CPUZ or SiSoft Sandra to determine your hardware.
To upgrade your CPU and video card you need to know what motherboard you have. Since the system is 3yrs old i'm guessing the video card is AGP and not PCI-e.
You state your CPU is a Intel CPU/3.4GHz single core. I'm gonna guess that its probably a CPU Socket Type LGA 775 with a Prescott core. If you do indeed have a Socket Type LGA 775 you can use any CPU with the same socket type, even if its multi-core. Here's a list -
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So basically the question is what can you cram onto your motherboard.
To be honest your video card and processor could probably last quite a while longer since most games still won't tax your hardware that much as long as you don't have to have ALL the graphical bells and whistles maxed. Your best upgrade for your $$ would be your RAM. Going from 512mb to a 1GB is a world of difference and going from 1GB to 2Gb is heaven. Anything after 2GB is just bliss and showing off lol!