Let me provide all information I know about upgrading your XPS, so you can choose the most appropriate to your needs.
About processor. Since you already have Phenom II X6 1075T the list will include only X6 CPUs, note that this is Dell approved ones
Here are the Processor Kits that Dell associates with the Studio XPS 7100 system:
* 1NDY0 AMD Phenom II X6 1035T
* M7P5K AMD Phenom II X6 1045T
* 68DKM AMD Phenom II X6 1055T
* YF80X AMD Phenom II X6 1075T
* 963RW AMD Phenom II X6 1090T
So, only 1090T is above your choice, not worth it. However, 1075T is Black Edition (unlocked multipliers) or not? I think it is - and if it is so, you can overclock it in this particular Dell. So, some improvement here is possible.
Memory. Memory can go up to 16GB, memory used is DDR3 1333 MHz, you can get improvement here.
I found out that this 460 PSU can support up to 6970 card (no matter what it said on minimum requirements)
http://en.community.dell.com/support-forums/desktop/f/3515/p/19395350/19924508.aspx#19924508
If you decide to upgrade you power supply. Most generic standard ATX power supply units can be installed, with either a 24-pin or 20+4-pin main motherboard power and P-2 12V 4-pin connectors, with or without the on/off switch. I recommend a good name brand unit though.
You should be able to buy a compatible power supply from either a local or online computer store.
Note: A power supply with five SATA power connectors, is needed.
Special note regarding size of ATX PSU. XPS 7100 chassis requires a standard-length (5.9-inch) ATX power supply. Some ATX power supplies, like the OCZ "Fatality" 550-watt, will not fit because they are too long and will overlie the two metal catches in the chassis, however, it is possible to remove the two catches with a grinder or other tool (I did it on my Inspiron 570, a little headache and no problems).
Check the length first before buying an aftermarket power supply for the XPS 7100.
Anything else, I missed?
So, based on this information, you can easily choose any components you wish.
If you are interested in overclocking, let me know, so we can discuss it here after you choose graphic card.