Just as your gaming computer, your server deserves good hardware. While at gaming, if your computer isn't recent enough for a new game, you'll get low FPS. Servers follow the same rule, but it will result in lag and instability.
This mini hardware guide will help you decide on what you need to build a perfect server and what you could use from your old computer to maybe form a mutant server from almost any computer. While HLDS may not fully use a computer's resources it will in the near future (hint: HLDS for Half-Life 2). While a 800mhz computer may be sufficient for a Counter-Strike server with no bots, it may lag with a few Condition-Zero bots. For those wondering, here are the minimum specs (according to me) for a no-lag 20 slot server: 1GHz Processor, 512MB RAM (DDR), 7200RPM drive, ADSL internet connection.
Having a good processor on a server is an important thing. Having an slow processor may cause slow information processing and will eventually cause lag. A processor together with the Hard Disk Drive's speed will determine the map loading time.
Your server won't be running 3D games while playing, but it will sure have a text console or a start menu. Without a hardware video card, your processor will be doing all the graphic calculations so make sure to get away from cheap graphic cards.
Having a lot of memory is very useful for HLDS, this is because each player's information is stored in the RAM. If there's no RAM left, Windows will start using swap ("virtual" memory on the Hard Disk Drive) and swap is really slooooooooooower than RAM. RAM's speed is also important so that HLDS could access the required information with less latency.
Hard Disk Drive (HDD)
No, a 300GB disk drive won't make HLDS run faster... The only specification that will help you out is the hard drive's speed, which is mesure in RPMs (rotations per minute). The hard disk drive's speed is important because it will determine the loading time of maps.
Power Supply (PSU)
A power supply must meet the high power demands of your server's hardware. Remember that it's recommended to buy a power supply with an integrated EMI filter for best reliability and longer life of all your components. You could also get an UPS so that your server could be online even during a power outage.
No need for fancy stuff here. Make sure the case can house your components and, if possible, choose a case which allows easy installation of fans near the hard disk drive slots. That way, you won't need to buy 5½" coolers for your drives.
Cooling your server is very important as your server will be running non-stop. Do not buy any passive coolers such as heat-piped Hard Disk Drive coolers. Make sure to have fans cooling your HDD and a fan near the CPU. Cooling will also influence reliability and life of your components. Filters on fans are also recommended so that you'll have less downtimes required for cleaning your server.