Are there any graphics cards uogrades for a Dimension 3100 desktop pc, also can I improve my CPU?
You have a PCIe video card slot, so you can upgrade to a PCIe video card. What version of Windows? XP? The CPU is a Pentium 4 that supports hyperthreading. What is your processors speed now? You should max out the system RAM to 2GB using 2-1GB sticks of DDR2 533MHz RAM. The power supply is 230 Watts, you may want to increase to a 350 Watt or higher
It has a PCI-Express X1 slot and 2 PCI slots but no X16 slot so graphics upgrade options are limited. Like Tmagic650 said the stock PSU is a 230w which would likely require an upgrade to handle a video card. The 915 chipset doesn't support any dual core CPUs so a P4 HT is the max it can handle.
Overall it might not be worth doing any major upgrades to this system since you could build a much more powerful one for around $200 (if you can reuse your current HDD and ODD). A modern system with current gen integrated graphics would easily outperform the best card that Dimension 3100 could support. You would be looking at around $100 for a video card and a decent PSU upgrade for the 3100 and more if you need to add ram or upgrade the CPU.