you can try a

Scaleo Pi 2662 - Core 2 Quad Q6600, 2GB, 500GB, see link below

Cheap Desktops UK - Fujitsu Siemens Scaleo Desktop PCs | Scaleo P | Scaleo H - Fujitsu Siemens Laptops, Notebooks and Desktop PCs | Amilo | Lifebook | Scaleo | Pocket Loox

They will upgrade the memory to 4gb should you need it .. Niether Premier Pro or Sony Vegas actually need a quad core processor.. in fact there is little or no software that actually does, but its a good future proof buy.

for editing your AVCHD i would go for Sony Vegas 7.0... I sometimes use it as an alternative for the sound mixing and fx features

good luck