Upgrade, replace, or build a new PC?

Okay so i posted on here a little while ago on overclocking my processor, Mainly because i think its bottlenecking my new graphics card. The CPU is a Q9300 2.5GHZ Quad-core, the video card is a Radeon HD 6870. The reason im posting again is that my PC is a Gateway FX7026 and i do not believe that it is not able to overclock... So now i am looking into buying a I5 but that means getting new RAM and a motherboard. (Mine supports DDR2, i know its old.)
So the I5 upgrade im looking at is about $300, and plenty of new PC's on Newegg are around $700, But they have all new components. So my question do you guys think its worth it to go for this upgrade, or look at a new PC? (If i look for a new PC i'll probably look at building one my self, its something i've been wanting to do.) Any thoughts are appreciated. :)
Note: I am pretty savy with computers, hardware and software.

My PC specs are.
CPU: Q9300 2.5GHZ quadcore.
RAM: 8 GB DDR2
GPU: Sapphire Radeon HD 6870
PSU: Corsair GS600 600wt
Hard drives: 2x360GB
Motherboard: Not sure, Not very good though, only has one PCI-e slot. :/

My motto is if you want the most bang for your buck, build your own computer

I agree with the above poster.

Anyways, if you're going with the "upgrade" route ... everything seems old except for the video card. I'd say keep your gaming peripherals (mouse, keyboard, monitor, speakers, etc) and just upgrade to the Sandybridge CPU (i5 has built-in OC). You'll probably have to upgrade your RAM and Motherboard as well, but that's about $300-500 versus a $700 PC that will only last you a few years.