blackhawk1928 :
I'd probably say the Intel X7460 or a W5590 or a 980X.
The X7460 and W5590 cost around 2-3k while the 980x costs around 1k. These are hexacored processors and currently the best of the best in the chain when it comes to multi-tasking.
This is very good answer to the question but you know its not practical and most probably not what the OP is looking for.
To Jeanne55,
With a 10 year old PC, you should replace all parts so it will be better to ask for a new build. Here is a good guide that will help you choose specs for your new PC:
http://www.tomshardware.com/forum/269162-31-recommended-builds-usage
Here is one of the lowest cost option you can find there:\
Bargain: $526.73
* CPU: AAMD Athlon II X3 435 Rana 2.9GHz Socket AM3 95W Triple-Core Processor Model ADX435WFGIBOX - Retail
* MOBO: MSI 785GTM-E45 AM3/AM2+/AM2 AMD 785G HDMI Micro ATX AMD Motherboard - Retail
* RAM: CORSAIR 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model TWIN2X4096-6400C5 - Retail
* HSF: Sunbeam CR-CCTF 120 mm Core-Contact Freezer CPU Cooler W/TX-2 - Retail
* HDD: Seagate Barracuda 7200.12 ST3500418AS 500GB 7200 RPM 16MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
* PSU: Antec EarthWatts Green EA-430D Green 430W Continuous power ATX12V v2.3 / EPS 12V 80 PLUS BRONZE Certified Active PFC Power ... - Retail
* GPU: SAPPHIRE 100254HDMI Radeon HD 4650 1GB 128-bit DDR2 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card - Retail
* DVD: Sony Optiarc 24X DVD/CD Rewritable Drive Black SATA Model AD-7240S-0B - OEM
* Case: COOLER MASTER Elite 330 RC-330-KKN1-GP Black SECC ATX Mid Tower Computer Case - Retail
* Thermal Compound: Comes with heatsink.
You can cut down the cost further if you modify it a bit:
HSF - Skip. Use the stock cooler that comes with the CPU.
RAM - Just get 2GB of RAM. It is a lot already. You can always buy more when you feels its not enough for your need.
GPU - Skip. Use the onboard video on the motherboard.
Some my suggest DDR3 RAM in this case you can get the MSI 785GM-E51 Motherboard and the DDR3 RAM.
There are a lot of good posters that will even give you most up to date specs and links to combo's that will give you good savings. You just need to tell them your specific budget and preferences.
I hope they come to your thread to help.