Advice Upgrading CPU

Not that long ago, I upgraded my graphics card from a Geforce 7600 GT KO to a BFG Geforce 9800 GT (thanks to the recommendation of some very smart people on this very forum). I'm very happy with it, but I knew when I bought it that it would bottleneck my system because of my CPU. Now however, I'm ready to upgrade my CPU as well. Unfortunately, this seems even trickier than upgrading my graphics card. I'm currently running with an AMD Athlon X2 Dual Core Processor BE-2300, with 2 GB of RAM, a 500W power supply, and Windows XP SP 3. I was wondering if anyone could recommend a new CPU, that would work with my chipset (a nVidia nForce 6100-405/430) and motherboard (an ECS GeForce6100SM-M), and not put me in the poor house. I was hoping to spend less than $300. I was also wondering if this was even possible and/or practical. Would a significant upgrade to a recent chip mean I would have to upgrade my mobo as well and thus (probably) my OS? Would it be more cost-effective just to build an almost entirely new machine (which I can't really do atm)? Thanks in advance for any recommendations and/or advice.

GIGABYTE GA-880GA-UD3H AM3 AMD 880G SATA 6Gb/s USB 3.0 HDMI ATX AMD Motherboard

G.SKILL Ripjaws Series 4GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9S-4GBRL

AMD Phenom II X4 810 Deneb 2.6GHz Socket AM3 95W Quad-Core Processor HDX810WFGIBOX

Not bad for $300 eh? You may have to reinstall windows but thats easy. Just make sure you have a backup of your data. Windows XP won't see the entire 4gbs of RAM but thats ok for now. Just upgrade to Windows 7 x64 Later down the road when you have the cash

If you want to spend a bit less than the other user's recommendation but get slightly better gaming performance, here you go-

Motherboard (I have it, very nice board for the price)- http://www.newegg.com/Product/Product.aspx?Item=N82E16813157176

RAM- http://www.newegg.com/Product/Product.aspx?Item=N82E16820231253

Socket AM3/Phenom II systems don't benefit much from anything faster than DDR3 1333 @ 9-9-9 timings, I have an article that I can post to prove it if anybody wants to argue :P

CPU- http://www.newegg.com/Product/Product.aspx?Item=N82E16819103808

BEAST of a CPU for $140.

Total cost of these parts is less than $266