You have several strikes against you with this computer. The specs you listed are extremely old. I'm actually surprised you can play at all with that small amount of memory.
In order to play games like CS: Source smoothly, you need to upgrade some stuff.
I recommend the following upgrades:
- Increase your RAM to at least 1 GB
-Change your central processing unit (CPU) chip to an Intel Pentium IV 2.6 GHz or above and throw away the Celeron chip.
- Upgrade to a PCI Video Card by ATI (Radeon) or NVidia (Geforce).
Assuming you have Windows XP, I have everything you need to do this upgrade if you are interested in buying the stuff from me. The price would be around $175.00 and your PC would run like never before.
The alternative is to buy a new PC.
www.tigerdirect.com has some outstanding deals for between $350.00 - $550.00. To do this right, I would spend the extra money for a new PC rather than upgrade the old one. It's just not gonna get you very far with the upgrades, but it will work for CS: Source if you choose to do so. Other more detailed games won't be able to run as well and you'll be in the same boat you are now inside of a year.
If you need help deciding on a new PC, please let me know and I'll help you out. Whatever you do though, stay away from Intel Celeron and AMD Semptron and Duron processors. These are very low end and don't do well for games at all.
Good Luck!