307-N07645 - £519 delivered
Intel® Core™ Solo Processor T1350 (1.86 GHz, 2 MB L2 cache, 533 MHz FSB)
Genuine Windows® XP Home Edition, SP2 (incl Symantec PC Restore Recovery)
15.4" Wide Screen WXGA (1280 x 800) TFT Display with TrueLife™
512MB 533MHz Single Channel DDR2 SDRAM (1x512)
80GB (5,400rpm) Hard Drive
Fixed Internal 8x DVD/24x CDRW Combo Drive With Software
6 Cell, 56Whr Lithium Ion Primary Battery
Intel® Media Accelerator 950 Graphics Up to 256MB shared graphics memory
Integrated Sound Blaster® Audigy® ADVANCED HD™ Audio
As you can see from the spec this is a 1.86Ghz solo cpu, but for just £35.25 you can upgrade to the 1.6Ghz Duo core cpu.
Also notice the screen, its the better spec Truelife one (thanks cralittlewood) which should be another £58.75 to upgrade to. Dell have made a mistake and priced it in reverse.
So with Quidco cashback and reducing the warranty to minimum this upgrade can be had for as little as £524 or less.