All depends on what your PSU capabilities are ,, but would suggest you get better than the GF8500, go for something like GF8800gt 512 or 9800GT but your PSU will have to be capable of running these cards minimum of 500W and 18Amps per 12volt Rails on PSU and have the PCI-E 6-8pin power connector for the PCI-E Graphics cards,
If your PSU is not up to par for them it may well be ok for a GF8600GT512mb, as HP dont have very good PSU.s in there systems.
I just upgraded the power unit from the stock 150-watt to a 575-watt unit, so I'm not too worried about power demands. The new power supply is set-up for PCI cards and SLI as well.
Thanks for the tips. Now I just have to see if your recommended cards are within my budget.