Intel® Pentium Dual Core Processor T2390 (1.86Ghz, 533 MHz FSB, 1 MB L2 cache) [Included in Price]


Intel® Core™ 2 Duo Processor T5750 (2.00 GHz, 667 MHz FSB, 2 MB L2 cache) [add £50.00 or £1/month1]


Intel® Core™ 2 Duo Processor T7250 (2.00 GHz, 800 MHz FSB, 2 MB L2 cache) [add £110.00 or £3/month1]


Intel® Core™ 2 Duo Processor T8100 (2.10 GHz, 800 MHz FSB, 3 MB L2 cache) [add £110.00 or £3/month1]


Intel® Core™ 2 Duo Processor T8300 (2.40 GHz, 800 MHz FSB, 3 MB L2 cache) [add £170.00 or £4/month1]

Intel® Core™ 2 Duo Processor T5800 (2.0 GHz, 667 MHz FSB, 2 MB L2 cache) [add £52,111.70 or £1,344/month1]
Intel® Core™ 2 Duo Processor T8100 (2.10 GHz, 800 MHz FSB, 3 MB L2 cache) [Included in Price]


Intel® Core™ 2 Duo Processor T8300 (2.40 GHz, 800 MHz FSB, 3 MB L2 cache) [add £60.01 or £2/month1]
I'm sure theres something wrong with the price for the CPU customisation price for the one in red.
Anyway you should have a look for these processors on ebay for example.
Lists taken from:
http://configure.euro.dell.com/dellstore/config.aspx?b=&c=uk&cs=ukdhs1&kc=NRS15251&l=en&oc=N0952504&rbc=N0952504&s=dhs
http://configure.euro.dell.com/dellstore/config.aspx?b=&c=uk&cs=ukdhs1&kc=NRS15254&l=en&oc=N0952507&rbc=N0952507&s=dhs