Two 9800GTs in SLI only require a 39 amp/700 watt PSU (for total system power), whereas two GTX 260 can get by on the same wattage, but need 50 amps of system power.
That said, I know the Dell XPS 630i uses a 750 watt PSU. I've also read a post of someone whom looked at his 630i PSU sticker and said it has four 18 amp +12v rails. The combined 12v rail amperage is what you use to determine if your PSU has enough current to run the GPU setup you're using. Not all PSUs are 80% efficient though.
Since Dell XPS models get pretty good components, warranty and customer service though, I'm thinking your PSU likely IS at least 80% efficient. If it is you'd have at least 57 amps on the combined 12v rails total current output (4x18=72x80%=57.6).
HOWEVER, that doesn't mean it's future ready with multiple Pci-E connectors as you have experienced. Many GPUs come with Molex to Pci-Ex adapters, some well made, some not. Avoid the ones that have one 4-Pin Molex on one end, and a Pci-Ex 6-Pin on the other. Reason being 4-Pin Molex plugs are only rated at 60 watts, and 6-Pin Pci-Ex at 75 watts.
There ARE adapters that use two 4-Pin Molex on one end, and a 6-Pin Pci-Ex connector on the other, but going that route would require your PSU to have 4 spare Molex connectors available, and it would be a bit cluttered hooking it up that way.
Sorry to say, but if the above scenario is not doable or to your liking, upgrading your PSU to one that has four 6-Pin Pci-Ex connectors, or 2 each 6-Pin and 6+2-Pin (an 8-Pin connector designed to detach two pins) could be tricky, as XPS systems tend to have proprietary 12v ATX connectors. The most common Pci-Ex connector configuration on PSUs is 4x6+2-Pin, because they fit a wide range of GPUs.
For a system using two GeForce GTX 260 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 650 Watt or greater power supply that has a combined +12 Volt continuous current rating of 51 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) @ 45°C to 50°C ambient temperature, is the most important factor.
The Dell 750 Watt OEM power supply, with its combined +12 Volt continuous current rating of 61 Amps and with two 6-pin PCI Express supplementary power connectors, is more than electrically sufficient to power your system configuration with a two GeForce GTX 260 in 2-way SLI mode. You will need to use two dual 4-pin Molex peripheral to 6-pin PCIe power adapter cables (usually included in the graphics card package) to supply the two missing 6-pin PCI Express supplementary power connectors from the power supply unit.