If you have a computer that meets all your needs, there is no need to upgrade. But 90% of your needs isn't 100%. If there is something your craving, such as playing a game at really high quality, or decoding movies fast, etc..., then maybe a new PC will be a good idea.
The Budget Build I linked you to totals in just under 800$ before taxes, before shipping. So with an OS, the total would be more-or-less 1000$. That Build is the absolute best bang on the dollar to the best of my knowledge.
If you are overclocking, the 1333mhz FSB CPUs will be a bad idea, because the multipliers wont have to be as high to get high clock speeds, example:
333mhz x4= 1.33ghz FSB
333mhz x2= 667mhz memory speed
333mhz x9= 3.0ghz CPU speed
266mhz x4= 1.06ghz FSB
266mhz x2= 533mhz memory speed
266mhz x11= 2.93ghz CPU speed
Higher multiplier = better overclocking. The multiplier can always be lowered on high(er)end motherboards, but it cannot be increased. If your not overclocking, ignore this entire explanation
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Quad core isn't nearly the improvement as dual core was compared to single core. Quad core isn't necessary, and it wont be for a while. Right now it's an excellent time to buy, Intel and AMD are having a massive price war. Memory prices are at an all time low. Harddrives are also amazingly cheap right now. A good motherboard can be had for 100$. The only thing thats missing it's AMD's DX10 videocards. However, the 8800GTS 320MB still has plenty of power in todays DX9 games.
You are not buying the 8800GTS for it's DX10 capability, but it's speed in current DX9 titles. Keep that in mind.