Sigh. Most of Panzer's post is a shot in the dark as to what can be had in this case.
Assuming the power supply in your system isn't some low blow cheap piece of tin with wiring in it (aka, no-name OEM stuff) you should be able to reconstruct the system nicely.
A triple core processor, especially one that is slower than the Athlon 64 X2 6000 isn't going to help things out. Yes, you added another core with that chip, but the 200 MHz frequency drop on an AMD platform isn't going to be helpful. If it were Intel, I'd be ok with it.
I've done the upgrade from the Athlon 64 X2 6000+ and it's a very powerful chip in its own right. There are two ways to go from here.
If you want forward thinking, upgrade the motherboard, CPU, RAM and Video card. If you just want to maximize performance, keep the CPU, throw in a new GPU and call it good. It really depends on what you want to do. We use a couple of the X2 6000+'s at my LAN parties as spare gaming systems, and they keep up just fine as long as they have a high end card in them.
Here's my recommendation on an upgrade, should you choose to go that route:
Since Bulldozer is just around the corner, there's no reason to not upgrade to an AM3+ motherboard if you're building new.
Newegg.com - ASUS M5A97 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX AMD Motherboard with UEFI BIOS
Newegg.com - AMD Phenom II X4 840 3.2GHz 4 x 512KB L2 Cache Socket AM3 95W Quad-Core Desktop Processor HDX840WFGMBOX
Newegg.com - G.SKILL NS 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Desktop Memory Model F3-10600CL9D-4GBNS
Newegg.com - PowerColor AX6790 1GBD5-DH Radeon HD 6790 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card
The graphics card alone is 10 times more powerful than your current card, and coupled with a high end AM3 Quad Core will likely push you through until Bulldozer prices come down to the point that you can upgrade if you want.
The pricetag for all of this? $370.96.
If your power supply is a no-name 650 watt unit, you'll want to get a name brand unit ASAP, expect to tack on another $60 for a good Antec or Corsair unit. Not bad considering how much they used to be. I wouldn't go with anything less than a name brand Power supply unit.
Panzer's approach is a little too conservative considering what you can get right now with more high end parts.
The parts I reference are on Newegg, so if you live outside of the USA, you'll have to let us know so we can adjust the parts/pricing for you.