Are you overclocking?
Do you need to buy an OS?
Here's a basic idea of what you can get w/in your budget.
I'd say start with this
Biostar P965 motherboard.
$105
Then take your pick out of these processors.
E4300
$175
e6300
$190
This
Kingston DDR2-667 should be alright for you.
$165
For a PSU, something on a non o/c machine w/o a high-end graphics card, 380W should be fine. However, I don't know how the quality of that Antec PSU is. I'd take a look at some of these. These are your better bets to stay w/in budget.
FSP 400W
$40
FSP 450W
$55
TOTALS:
cheapest route: $485
more expensive route: $515
Either way, you're well w/in budget to select a better video card...
Since you don't game much and a 6600GT has been getting you by, a 7600GT might be a nice upgrade for you:
eVGA $90 after rebate
A 7900GS is even faster
XFX $145 after rebate
And better yet is an x1950Pro
Diamond $170 after rebate
Depending on what you choose and how you base your system, you could be able to upgrade any of the above components.
What I have above is a point to get you started on, add or modify whatever you want. Post again when you have something more concrete configured.