Check out newegg.com for some good prices on stuff. All the prices I will list in here will be from that site.
If you get a 754 board get a 64 2800 for around 120$, it better than all the Semprons and its cheaper than some of them. The 64 3000 for 939 are around 150$ and those boards cost abit more, just throwing that out there. For socket A get a mobile barton, the 2400 is 80$ max and with PC3200 its easy to OC since those can easily run at with 200mhz FSB as oppose to its original 133mhz, and the multipliers are unlocked also. Plus Socket A boards are cheaper than even 754 boards!
As for the graphics I don't realy know. The nvidia 6600 normal is kinda pricy for budget, but it is a VERY good deal, 103$ at the lowwest end for PCI-E and 113$ for AGP, and it can OC as well VERY easily. Anything less than that would have DRASTICALY lowwer performance, and would not cost that much less. But the next step down would be about an ATI 9600 or nvidia 5500, the 9600 would be a better option. Besides those go for onboard.
Looks like everything else you have should be fine, just find a board for the CPU and Graphics you have decided on.
OPSE! Didn't notice you were AUS untill I finished
Well.. the AUS dollar is about .75 that of the US dollar, so you would need 1.33 AUS dollars to = 1 US dollar if that helps any. Also the international shipping will cost more probably

ECS 755-A2 MoBo | 754 NewCastle A64 3000+ | 1.5 GB PC3200 3-3-3-8 | 80 + 250 gig HD 7200 RPM SATA | BFG Geforce 6800GT | Audigy 2 GAMER | 550 WATT PSU | SyncMaster 712n | Altec Headphones | Logitec 5.1 System