2 problems. Dell PSU's are cheap nasty things and it's highly likely the 12v rail does not come close to 305w, I'd be surprised if it was over 150w. What amps does it say it can deliver on the 12v rail? It should be on a sticker on the PSU.
Your other problem is you need to look up you exact model Dell. Dell have been known to block GPU upgrades by making the BIOS only allow certain specific models of gpu.
Dell really don't want you upgrading there machine, they would rather you bought a whole new PC