Whoa, that's the classic sales line you get at the stores! I mean, get the great processor, the rest you can upgrade. Doesn't work out like that.
What happens is that the customer goes home with a processor that cost too much, more than they need, with lagging performance because the supporting parts were generic or lower-grade. When they go to upgrade, whoops, it turns out that the packaged computer from Best Buy or Circuit City or similar stores (not sure what you've got in Brisbane) used wholesale components from Taiwan or Hong Kong that cannot be swapped without huge compatibility headaches.
I've tried it on HP desktops and Dell laptops. Unless you're building your own, and picking each part for future compatibility with after-market upgraded parts, don't plan on upgrading. For a pre-built box, that's going to be the box you'll have until you're done owning it.
The short and sweet, I promise you these shopping priorities will keep you out of trouble and you'll be happier with the new box:
Average processor for a good price (last year's most popular chip or older), maximum RAM for speed (8GB), maximum video card for games and sketch-up, ignore the rest.