It depends on the processors. GHz are not all that matters. If you are comparing two identical processors where the only difference is the clock speed, then 3.2 GHz will provide approx. 20% performance gain.

GHz is the clock rate of the processor, in otherwords, the number of cycles per second. The processor will execute commands every cycle so obviously, the more cycles per second, the more often commands will be executed every second.

Without knowing the price of the 2.66GHz processor, we really can't say if $490 is worth it or not. Nor can we say if it is worth it for you because we do not know what you will be doing with the computer.
 
Here the difference installed in a laptop is $285

http://www.pro-star.com/index.cfm?mainpage=serial-price&serial=D900F&promoteid=90000314&screenid=13067115&cpuid=30111400&hdid=40105519&cdromid=20074658&ramid=80111701&batteryid=10031871

You are basically choosing between an 920 and a 960

$288 http://www.newegg.com/Product/Product.aspx?Item=N82E16819115202
$589 http://www.newegg.com/Product/Product.aspx?Item=N82E16819115224

You can simply overclock the 930 to 960 speeds though it must be noted that you'd be able to OC the 960 even higher.

That's a 20% overclock so you'd want at least DDR3-1333 which apparently is unavailable from Dell which is why I'd recommend building your own system.

Looking at The Studio XPS 9000 for $1299

Genuine Windows® 7 Home Premium, 64bit, English ($104)
Case and PSu ($109)
MoBo ($159)
Intel® Core™ i7-920 processor ($288)
8GB Dual Channel DDR3 SDRAM at 1066MHz - 4 DIMMs Bonehead Mistake....920 requires triple channel ram ($145)
750GB 7200 RPM SATA Hard Drive ($60)
ATI Radeon HD 5450 1GB DDR3 ($52)
20.0" Dell ST2010-BLK HD Monitor with VGA cable ($124)
16X DVD+/-RW Drive ($20)
Dell Studio Consumer Multimedia Keyboard
Dell Studio Optical Mouse ($20)

That system can be built of Newegg for $1081.

Here's an alternate:

Antec 900 / EA-650 ($180)
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129064

Asus P7P55D-E LGA 1156 Intel P55 SATA 6Gb/s USB 3.0 ATX Intel Motherboard ($160)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131620

Intel i7-920 ($289)
http://www.newegg.com/Product/Product.aspx?Item=N82E1681911520

Mushkin Enhanced Blackline 4GB (2 x 2GB) DDR3 1600 (PC3 12800 Model 996782 ($105)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226103

Seagate 7200.12 1.0 TB ($95)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148433

XFX HD-585A-ZNFC Radeon HD 5850 (Cypress Pro) 1GB ($300)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150442

ASUS VW246H Glossy Black 24" 2ms(GTG) HDMI Widescreen LCD Monitor ($229)
http://www.newegg.com/Product/Product.aspx?Item=N82E16824236049

SAMSUNG Internal 8x Blu-ray read 16X DVD Write Combo Drive - Model SH-B083L/BSBP w/ LightScribe ($100)
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151199

Saitek CD6400000A1/06/1 Black Modern Warfare 2 Combat Keyboard ($40)
http://www.newegg.com/Product/Product.aspx?Item=N82E16823175007

Saitek CD6443000002/04/1 USB Wired Laser 3200 dpi Modern Warfare 2 Sniper Gaming Mouse
http://www.newegg.com/Product/Product.aspx?Item=N82E16826102042 ($42)

XIGMATEK BALDER SD1283 120mm Long Life CPU Cooler ($45)
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233040

OCZ Freeze TIM ($6)
http://www.newegg.com/Product/Product.aspx?Item=N82E16835202006

That's $1591 for a machine that has a much bigger monitor capable of HD resolution, includes Blue Ray player, and a GFX card capable of playing the newest DX11 games at HD resolutions, a larger HD, much faster memory and can be easily overclocked to 4.0 Ghz. That's $200 cheaper than the Dell w/ a 3.2 Ghz which basically would have trouble playing any game on a 20" monitor. If ya not into gaming, grab a 4100 card and sit with the rest.