If you wanna run a source engine game like CS:S make sure the notebook you get has a pretty good processor because it's weighting on the performance seems to be greater than the video card in my opinion. You also have to watch out for notebooks with Intel GMA video cards because those suck balls, trust me. At least have one with a NVIDIA or ATI video card since they're the leading companies for them so far.
I don't know any great notebooks for 600 dollars or less but if you're willing to pay 200 dollars more you can get this notebook
http://www.ibuypower.com/ibp/store/configurator.aspx?mid=295
There's also a section on the website where you can customize it to your likings and here's it's current specs.
Processor Support Intel® Merom FSB800 CPU T5450/T8100/T8300/T9300/T9500
Chipset Intel® PM965 + ICH8-M
Graphics & Video Module Mobility 256MB NVIDIA® 8600GT PCI-Express 3D Video
Main Memory DDR2 667 SO-DIMM x 2 slots, MAX: 4GB
Display 15.4" WXGA ( 1280 x 800 )
PC Cards Slot x1 Card Reader SD/MMC/MS/MS-Pro 4-in-1 Card Reader
Hard Disk Drive Supports SATA HDDSupports SATA HDD
Optical Drive Build-In DVD+/-RW
LAN/Modem Realtek Gigabit LAN + 56K Modem
Wireless LAN Supports Intel® PCI-Express mini card 4965 & 3945
Built-in CMOS Camera 2.0 Mega Pixels
I/O Ports • USB 2.0 : 4 • IEEE 1394 : 1 • MIC-in : 1 • Headphone out : 1 • RJ11 : 1 • RJ45 : 1 • TV-Out : S-Video x 1
Audio Realtek ALC 888H
Battery Pack & Life Li-ion battery
AC Adapter 20V DC, 110~240V AC, 50/60Hz universal
Dimension 355mm(W) x 257mm(D)x 35.2mm(H)
Weight 6.6 lbs
You can also check a list of cheap notebooks from newegg right here from lowest to highest.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2030260032&bop=And&Order=PRICE
If you don't want the notebook mailed to you, you should find a pretty good notebook at Best Buy if you have one near you, they usually have a good deal and some great packages. Although their customer service sucks so stay away from it.