or even a GTX 460 if he is a nvidia fan
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127519
that particular cpu upgrad is not advisable.
Overclocking your GTS450 is only going to net you 3-4 fps in dirt 2. as you Evga is probably already slightly OC'd. see Tom's article:
http://www.tomshardware.com/reviews/geforce-gts-450-overclocked-roundup,2762-16.html
Lets look at the GTS 450 SLI option (evga gts450 is going for $130 on newegg.com):
http://www.tomshardware.com/reviews/geforce-gts-450-gf106-radeon-hd-5750,2734-4.html
this article says that's the way to go, but.... it was on a system that had 5 times the CPU you have. They did say it should be effective in a i5-750. which only about twice as fast in gaming performance to your E7500 Wolfdale.
quad duo 9505 =$240 + $130 for a GTS 450 = $370
For only $80 more on newegg.com (maybe less if live near a Microcenter):
you get a Sandybridge build for under $450 (this a MIR of $20 so its even less):
Intel Core i3-2120 Sandy Bridge 3.3GHz LGA 1155 65W Dual-Core Desktop Processor BX80623I32120
Item #: N82E16819115077
$149.99
Mushkin Enhanced Essentials 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit
Item #: N82E16820146748
$39.99
BIOSTAR TH67+ LGA 1155 Intel H67 HDMI SATA 6Gb/s USB 3.0 Micro ATX Intel Motherboard
Item #: N82E16813138296
$104.99
MSI N460GTX Twin Frozr II SOC GeForce GTX 460 (Fermi) 768MB
Item #: N82E16814127519
$20.00 Mail-in Rebate
$149.99