I agree with Cameron on this system. I have overclocked old systems to see what performance gain there was and its very minimal. If you had an old Pentium 4 HT and needed to play a game that called for a Pentium D minimum and you weren't going to use the system for modern gaming and weren't going to invest more than like $15 into it, I'd say push the Pentium 4 HT from like 3.0Ghz to 3.3Ghz and try to get the Pentium 4 HT 3.3Ghz with overclock to perform similar to a Pentium D 2.8Ghz. I did this long ago to get Need for Speed - Carbon to run better on a Pentium 4 3.0Ghz HT system. CPU temp would climb and hold at 63C with this overclock and new thermal compound with massive heatsink with heatpipes. I had the thermal shutdown temp set to 80C and highest seen was 74C on a hot summer day when the air in the home was warmer. For the fact that I gamed mostly evenings when the air was cooler it wasn't much of a problem.
For around $100 you can get a barebone lower end but far more powerful than a 2001-2005 computer for processing power Motherboard, CPU, and RAM.
Not promoting this build below as the way to go for a gaming setup, but it did work as a gaming setup for my wife for 2 years prior to getting her a Core 2 Duo E6600 system.
Motherboard CPU Combo Deal at newegg for $65 ( Biostar Socket AM3 Motherboard and Sempron 140 2.7Ghz single-core CPU )
4GB DDR3 RAM - $25
Prior to this she was running a Pentium 4 2.8Ghz HT. This older system benchmarked with a CPU score of like 399. The new low end CPU scored 780 for performance, so it was almost 2 x better than the Pentium 4 that she had. It was also very noticeable that it performed better and faster than the Pentium 4 as well which was very surprising for an at the time modern low end budget single-core CPU. I ended up moving her GeForce 8800GT video card from the Pentium 4 to the new build and she was able to game and gaming performance was better than ever for her.
She used this for about 2 years until she started to heavily multitask on it and the Sempron 140 single-core CPU wasn't very good for having many things going at the same time. She played World of Warcraft on it and it played with no lag, but when she has iTunes going, plus browser with many tabs and Facebook running in which Facebook has many CPU hungry background scripts running, and then tried to game, it brought this single-core to its knees. I was given a HP Pavilion tower that was someone elses junk computer that got hit by lightning. I was going to scrap it for reuse of its case, but found that the 56k modem is all that was fried and the modem that was cooked was keeping the motherboard from successfully posting. Removed modem and it was a healthy system. Let the person who I got system from know that I fixed it and do they want it back. They already bought newer system and said I could still keep it. I then upgraded the Core 2 Duo E4300 1.8Ghz 2MB Cache to a Core 2 Duo E6600 2.4Ghz with 4MB Cache that the board supported and I had this CPU not used in a ESD baggy. She has been using that Core 2 Duo for the last 2.5 years with no problems in performance, but that older Core 2 Duo E6600 is far better performance than the Sempron 140.
Getting back to the Sempron 140, I ended up repurposing this system as our HT PC to play movies and streaming content to the TV with HDMI off of the video card. The Sempron 140 was plenty for quick loading and crisp and lagless streaming of Netflix and movie files.
