What should I upgrade?

Hi! This is my first time here, and I hope I like what I find!  

Anyways, I'm looking for higher frame rates in newer games on 1680x1050.

Here's my current setup:

RAM:
CORSAIR XMS2 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model TWIN2X4096-6400C5

Motherboard:
GIGABYTE GA-E7AUM-DS2H LGA 775 NVIDIA GeForce 9400 HDMI Micro ATX Intel Motherboard


Processor:
Intel Core 2 Duo E8400 Wolfdale 3.0GHz LGA 775 65W Dual-Core Processor BX80570E8400


Video Card:
XFX HD-487A-ZWFC Radeon HD 4870 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card

PSU:
500W came with my case

Here's what I was looking at to upgrade:

G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-4GBRL
BIOSTAR T5 XE CFX-SLI LGA 1156 Intel P55 ATX Intel Motherboard
Intel Core i5-760 Lynnfield 2.8GHz LGA 1156 95W Quad-Core Desktop Processor BX80605I5760

OR

ASUS EAH6850 DirectCU/2DIS/1GD5 Radeon HD 6850 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity

Which would be the better upgrade for better frame rates in games? For example, GTAIV I get horribly low frame rates, and in NFS:HP I get decent FPS. In fallout, my framerates are fine.

Are the parts I chose good, or do you have anthing batter in mind? I'd prefer Intel, but with GPU I don't care who its made by.

Budget: Under $500