|L2 cache is the least of your problems.|
Your system is somewhat outdated but the biggest deterrent to better performance is the Intel motherboard. If you had an overclockabled aftermarket board (Asus, Gigabyte, MSI, etc), you'd be able to make your system scream without even swapping the CPU.
For example, changing the E7500 clock settings from it's defaults of 11 x 266MHz (2.93GHz) to say, 8 x 400MHz (3.2GHz), would not only increase the FSB speed by 50%, but it would allow you to take full advantage of your DDR2-800 memory. You'd be running the CPU:DRAM freq ratio at the desired 1:1. Right now, your RAM & CPU are out of sync, running at the less than optimal ratio of 2:3.
If you're not going to change the motherboard, the Q8400 is the CPU to go with. And get a decent video card too. Your last post you asked about the onboard X4500 vs the 8400GS...they both suck for gaming.