I see that this model is some 10 years old. Forget about upgrading the CPU, or anything, although - if you really are attached to this machine and it runs Windows 7 or higher - you can consider to add an SSD for a faster boot (don't do it if you still use the Windows Vista it came with).
Buying another PC is the only feasible option.
Intel 2 Quad CPU Q6600 2.40GHz
Nvidia GTX 545
I am having some CPU bottle necking while playing games I think, and I'm not sure what I can safely upgrade too.
Would appreciate some recommendations. Thanks!