Unless you are 100% certain that your workflow demands a quad, a fast dual core like the
AMD Phenom II X2 545 Callisto should work nicely (as should the Radeon HD 5670 video card in that combo).
nVidia GPU processing in CS5 is generally associated with the acceleration of specific color correction features so, once again, do your homework.
Lightroom can use a quad to full benefit if you import/export bunches of images. There are also plug-ins that will put 4 cores to good work. Otherwise, a couple fast cores will keep PS smokin'
Here is an
Asus M4A79XTD EVO 790X for $74 after rebate -- great motherboard for a great price.
Snag a 8GB (2 x 4GB) kit of DDR3 1333 for $80-$90 and a Spinpoint F3 HDD. Using that Spinpoint F3 as a scratch drive with an SSD for your OS/Apps will 'pop' if it works in your budget.
With a $35 after-market HSF you should be cruising along at 15x250MHz with the IMC/NB at 2500MHz. Should be nice and snappy