As previously commented by epixoip the following unit will not be capable of support the various GPU's available due to power requirements.
There is x16 slot/riser card option but would not help much.
http://h18000.www1.hp.com/products/quick...ocid=13595
There is x16 slot/riser card option but would not help much.
http://h18000.www1.hp.com/products/quick...ocid=13595