I've tried googling some of your components, and it looks like your single processor is only 2GHz which is really really out of date - like half a decade or more. Your graphics card is also holding you back, I googled "GeForce 6150SE nForce 430" and the results coming back are telling me that you could be using an onboard card instead of a discrete graphics card (which is a must have with gaming).
Overall, it looks like you may be at the end of your lifetime of switching out components for another. While PCs pride themselves over laptops by offering the ability to upgrade/customise, there is a limit to this - and this is your motherboard. Everything plugs into your motherboard, such as your processor, graphics card and RAM. For example, if you have an old motherboard, it stops you from installing more recent developments of processors, graphics cards and RAM chips.
Even though I can't tell what model of motherboard you have, the specs you have given me tell me that you are likely at that point where the very last component you need to switch out and upgrade IS your motherboard. However, newer versions of motherboards can only support older-specification of graphics card, processors, RAM etc by so many months/years. This means if you replace your motherboard, you will effectively be looking at replacing your entire system, equivalent to getting a brand new computer.
You could hunt on newegg or a commercial retail store for a new 'tower' (aka: a completely new box set up for you) which is fine. However, if price is a huge issue then an alternative option is to visit one of the smaller stores and get a quote on how much it would cost if they made a set up for you. You could bring in your current PC and let them know there are some parts you would like to keep or re-use (things such as chassis fans will save you a few dollars but not much, and you might be lucky enough to keep your hard disk to save you another $30+ if you format it first, and if you have a DVD optical drive then that's another saving, you might also be lucky enough to keep your power supply). EDIT: You may also be able to keep the case.
Regardless, you made the choice years ago of choosing a PC over a laptop, and in doing so you also gave yourself the ability to save money when it comes to upgrades ;-)
Don't forget to check back to the Games, Gaming and Hardware forums if you need to reply. They are also very friendly and helpful in the sense that they will give you awesome opinions on your purchase; so if you get that quote or if a particular premade model is to your liking, they will let you know if you're about to make a good or bad purchase.
EDIT: You may also be able to keep the case.
Edited by Moonrage on 2/21/2012 2:19 PM PST