Fastest PCI-e card that doesn't require external power?

Last post similar to this was back in 2010 and I didn't want to necro that one. Got a XPS 420 with a Q6600, Win 7 64-bit and the original POS Radeon HD2400 256MB PCI-e card in it. Looking for opinions/suggestions on the fastest card that doesn't require any external power. Pretty sure just about anything current will be hella faster. Brand/Team doesn't really matter, price not so much either.

You could probably get away with a 7750 or 7770, even if you have to use a molex -> PCIe adapter. Maybe even a 7850.

How many SATA power connectors do you have? I know Dell doesn't do molex connectors but if you have 2 unused SATA connectors you can buy some adapters that turn them into a single 6 pin PCIe power connector.

Selection is very limited without ANY power connectors, you're looking at 1 notch above the bottom level card which is still pretty slow.

I think these are the most powerful cards available from either side without any connectors:

nVidia: GeForce GT 545
AMD: Radeon HD 7750

this Molex to PCIE Power Adapter

or

this SATA to PCIE Power Adapter

will make it so that non powerd is not an issue.. then the only issue would be getting something within your psu's power specs.

If you have no avalible molex or sata plugs you can buy splitters that split one connection into two in order to get the connections you need.

As the guy above said. without power your options are going to be severely limited.

I think the constraint is that like most big name OEM computers it only has a power supply big enough for the computer as shipped. The Dell in this case I think comes with a 375w PSU and from all accounts the 12V line is not up to the task of anything beyond what it came with. However apart from some of the cables being generously long it seems to be a standard ATX power supply, so it could be upgraded to power a better card.

Thanks for the replies. The 7750 looks good. +1 to everyone.