CPU and GPU upgrade ideas?

Hey, I need some help here because I honestly don't know the first thing about computer parts.
Basically, I'm rocking
4GB of RAM—two 2GB sticks, obviously
A 2.66 Intel dual core, two cores total
A 500GB hard drive
To save time, here's the full breakdown: MS Windows 11 Pro 64-bit SP1
CPU
Intel Pentium Gold @ 2.60GHz 44 °C
Wolfdale 45nm Technology
RAM
4.00 GB Dual-Channel DDR3 @ 399MHz (6-6-6-15)
Motherboard
ECS G41T-M7 (CPU 1) 30 °C
Graphics
SyncMaster (1280x1024@60Hz)
ATI Radeon HD 5450
Hard Drives
Seagate BarraCuda Hard Drive 40 °C
Optical Drives
DTSOFT Virtual CdRom Device
Optiarc DVD RW AD-5260S ATA Device
Audio
Realtek High Definition Audio

So, someone suggested I swap in a Core 2 Duo and grab a 4GB graphics card, specifically an
MSI Nvidia GeForce GT 730 ddr3
But will I actually see any difference if I do that? Or what else should I be looking at? I really need some advice here—I'm just trying to get enough juice to play better games.

That GPU is pretty much junk. Honestly, you probably need a quad-core for modern games; sticking with a dual-core just isn't going to cut it anymore.

CPU:
http://www.ebay.com/itm/processors/intel-processor-listing-link

GPU:
http://www.newegg.com/p/graphics-card-details-link

Or, you might get a better deal looking at used parts—this looks like a solid offer:
http://www.craigslist.org/video-cards/deal-link

One last thing, though—you should probably let us know which power supply you're running (the exact model on the box or label). This new setup is likely going to pull three or four times more power than what you've got now.

Hey, let me know what you think the best move is here. I'm definitely looking to stick to used parts to save some cash.
If you're familiar with eBay or Craigslist, just shoot over some links for what I should grab. My budget is pretty tight—maybe around $100, maybe stretching to $120 if I have to.

http://www.example.com/article/core-2-quad-q9400/

http://www.example.com/article/sapphire-6850hd/

I guess you didn't mention which power supply you're running.

I don't know where it says that exactly, but there's a sticker right here on my keyboard that says 600w maxx
Mind giving me a quick rundown?