I'd highly recommend just to upgrade the motherboard and video card, possibly RAM. Your CPU will do fine for now, and it'll be well worth it to go for Deneb or Propus.
I'd probably go for
* DFI 790FXB
board (note the B, which has an SB750 southbridge),
* 4870 1GB
* 4 or 8GB DDR2-1066
(I have 8GB of the same type) - though you'll only be able to run it at 1066 speeds with a Phenom. But with your X2, it'll leave plenty of room for HTT increases.
Out of curiousity, what PSU are you using?
Originally Posted by SiZcO
Between the X4's and Phenom whats the real big difference?
the Athlon X4's will be based on the Propus core, which is the same as Deneb (Phenom II), but without L3 cache
I'm not sure if Propus will have unlocked multiplier versions, however. And we have yet to see how well Deneb/Propus overclock using HTT increases.
Oh, and @ Belzybub:
i'm on a set budget of about 500-600 bucks