I could do panos with up to 700 images with 8GB ram (very slow) and with 16GB still slow but worlds faster than with 8GB....
so 72GB should be a very solid basis...
a good GPU card and you've a hopefully cheap server...
I would go for the nvidia titan with 6GB GPU http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-titan
if money does not limit you, go for 18x8GB reg.memory. a good one is available for 140â‚¬ for 1x16GB.
a not uninportant question is which drives you will use.
I think dells come prefonfigured (the xps comes with 2xHD as RAID0 for the OS drive... lol...make sure this is not the case for your dell)
so if possible put the OS on a RAID1 made by 2 SSDs
for images, temp dir, destination dir put some 6GBit HDs into your dell using RAID5(3+ disks) or RAID0(2+disks if speed is prefered and data loss less important)
for archiving your done work and sourceimages:
if money is no issue than you could use the thecus NAS with 40TB with 10 HDs and with up to RAID 60 for performancehttp://www.thecus.com/product.php?PROD_ID=73