GIGABYTE GA-MA78G-DS3H AM2+/AM2 AMD 780G HDMI ATX AMD Motherboard
$86.99
AMD Athlon 64 X2 5600+ Brisbane 2.9GHz Socket AM2 65W Dual-Core Processor Model ADO5600DOBOX
$69.00
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MSI P43 Neo3-F LGA 775 Intel P43 ATX Intel Motherboard
$84.99
Intel Pentium E5200 Wolfdale 2.5GHz LGA 775 65W Dual-Core Processor Model BX80571E5200
$82.99
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SAPPHIRE 100245L Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
$149.99
Hanns·G Hi-221DPB Black 22" 5ms Widescreen LCD Monitor
$159.00
Kingston ValueRAM 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model KVR800D2N5K2/2G
$24.49
AMD Build: $497.72 (after Shipping)
Intel Build: $504.26 (after shipping and $10 MIR)
For gaming these two will perform about the same. In other areas the Intel CPU wins hands down in overclocking potential, and energy consumption. Only cons being the LGA775 platform will eventually wither away in favor of the Core i7 platform, so you might find yourself wanting to upgrade your CPU and no place to get an upgrade other than eBay, whereas your chances of getting an upgrade down the road with AMD is greater with their continued "support" for the AM2+ platform which will wither and fade in favor of whatever is after the AM3 platform (as AM3 processors are supposed to be compatible with AM2+).
That's if you really want to get a new system though. Personally, your system is fine the way it is. If I were you I'd throw my money at a
4870 1GB/
GTX 260 and a bigger monitor. Then upgrade down the road again to something better. You'll probably see a bigger jump from a better video card then upgrading most of your system, plus it's more fun upgrading down the road I think. =)