Please help with PC upgrade

Hi
I would like to upgrade my PC to a point where I can play modern PC games with a 1680x1050 resolution smoothly.

This is the current state of things:

Processor:
Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz (2 CPUs)

Motherboard:
ASUS P5K PRO

RAM:
4GB DDR2-1033 (2 modules of 2GB)

Graphic card:
ASUS NVIDIA GeForce 8800 GT

Power:
Thermaltake Toughpower 650W

Screen:
HP w2207h

OS:
Windows XP (32)

After some reading I have decided the following.

First I will upgrade the graphic card to:
SAPPHIRE Radeon HD 7870 GHZ Edition

If that doesn't produce the desired effect, then I upgrade the rest to:
OS:
Windows 7 (64)

Processor:
CPU Sockel 1155 Intel® Core™ i5-3450 FC-LGA4, "Ivy Bridge"

Motherboard:
GIGABYTE GA-H77-D3H

RAM: 8GB DDR3-1600 from Corsair or GeIl (don't know which one is better)

Now the question:
What do you think is better for my motherboard-processor combo? 2 or 4 RAM modules? I have tried to find some information about this without luck and I don't have the "know how". Theoretically a 4core processor goes with 4 modules of RAM, but there is also the motherboard connectivity between both that matters, right? Or is all this irrelevant?

To follow my line of thought (I repeat this is without any knowledge, its pure logic and I could very well be talking complete nonsense. That's why I ask you guys): ^^
Processor core (=house) ---- motherboard connection between processor and RAM (=highway) ---- RAM (=working place)

ideal:
[c1]----------[R1]
[c2]----------[R2]
[c3]----------[R3]
[c4]----------[R4]

traffic jam:
[c1]----------[R1]
[c2]----------[R2]
[c3]----------
[c4]----------

[c1] [R1]
[c2]----------[R2]
[c3]----------[R3]
[c4] [R4]

I thank you kindly for the help. :-)

R