That's a pretty weak older dual core with no hyper threading. You're not going to be able to go very high on GPU performance with that CPU or it's going to severely bottleneck. It looks like you can install a regular full height GPU card but you will want to measure the distance from the where the card meets the back panel of the case to the hard drive cage to verify clearance as some cards may be too long.
For the most part this shouldn't be an issue for you, as you can't utilize a very high end card that's likely to be that long, but some lower tiered cards can be long too. I think it's likely anything beyond 8"-9" will be too long for your case but verify the measurement yourself.
Most likely the best options for your configuration that can be run without losing the full potential of the card are going to be either a GTX 750 TI, GTX 760, R9 270X or HD 7950. Your system might even struggle with three of those four, the 750 TI should certainly be fine, since you've only got 4GB of RAM. You can add another 4GB of RAM to your system to add a little gas to the tank.
More important however is you power supply. If you still have the OE power supply that came with the unit, which is likely, it's not going to be sufficient for any of those cards. Plus it likely won't have the necessary PCI power cable needed to power an aftermarket card.
You need a Tier 3 or higher (Preferably Tier 2B or higher) PSU between 450-500w. This would be a good configuration for your unit but don't expect to be able to play recent titles at high or ultra settings. You're never going to be able to do that with your system since your card selection is so limited by your CPU. It should play most titles at medium settings though, with a few very demanding titles perhaps needing to go a notch lower.
PCPartPicker part list /
Price breakdown by merchant
Video Card: EVGA GeForce GTX 750 Ti 2GB Superclocked Video Card ($129.99 @ Newegg)
Power Supply: Antec 450W ATX Power Supply ($34.99 @ Amazon)
Total: $164.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-10-22 18:02 EDT-0400
And if you wanted to bump the RAM you would need to either remove the two 1GB modules you have installed and install three new 2GB modules (Your MB only supports modules up to 2GB in size.) or you can leave it and install another 2GB module for a total of 6GB of RAM, which is probably the better option financially.
PCPartPicker part list /
Price breakdown by merchant
Memory: Kingston ValueRAM 2GB (1 x 2GB) DDR2-533 Memory ($38.00 @ Amazon)
Video Card: EVGA GeForce GTX 750 Ti 2GB Superclocked Video Card ($129.99 @ Newegg)
Power Supply: Antec 450W ATX Power Supply ($34.99 @ Amazon)
Total: $202.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-10-22 18:07 EDT-0400
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