Yes, I think your cpu is at fault.
The E5700 was a good cpu in it's day. It had a passmark rating of 1763.
I suppose you could buy a used quad core Q8400 on ebay for about $50 if your motherboard supports it.
It will have a passmark rating of 3244.
Today, the slowest and cheapest $50 intel cpu, the G1610 dual core has a rating of 2551.
Most games depend on fast individual core speeds, so I don't think upgrading to a quad on your lga775 motherboard will be all that effective.
I think you need to plan on a new build.