Which power cord to upgrade first?

I am just starting to explore the world of PC upgrades, and am wondering what a typical upgrade strategy would be. Which cable upgrade would have the biggest impact: my digital source or my receiver? Should I eventually do both?
Your amplification typically will see the biggest improvement in a power cord upgrade. I would recommend giving Frank at Signal Cable a call and discuss your situation with him. You can try his cords for 30 days with a nice return policy.
I don't think there is a "right" answer. In my case I find it to be on the DAC and preamp. My advice would be to buy used and experiment.
Amplifier OR amplifier AND power conditioner (if you're using a power conditioner.) These two cords should be no less than 10AWG wire size. A floating shield (connected at the AC plug end only) is a nice option, but most modern equipment has internal RFI/EMI filtering.

Front end stuff can use smaller gauge PCs, but no less than 14 AWG.
Exception: DACs seem to benefit from larger conductor sizes like 10 AWG. They're not power-hungry devices, so I don't know why this is, it just is.

Any device that has digital circuitry of any kind (DACS, CDT, CDP, HT processors, etc) should definitely have a shielded power cord, and preferably use a circuit that is separate from other (non-digital) front end stuff (tuner, tape, preamp, phono preamp, etc.)
I've never heard a bigger difference than when changing the PC on the source, much more so than the amps in my experience. Obviously, others have heard greater shifts elsewhere. As Wellfed suggests, buy used and experiment.
In my system,the source (CD player) cord upgrade had the greatest improvement.
The power amp and preamp cord improvements were subtle, the CD players improved cord was substantial. From what I read, the CD player can 'broadcast' digital noise through the power cord corrupting the other components. The improved cord keeps the digital noise from polluting the rest of the chain, along with the the improved power delivery.

CD player = Musical Fidelity A5 with JPS labs digital AC
Preamp = Adcom GTP750 with VHAudio flavor 2.
Amp = Conrad Johnson MF2250 with VHAudio flavor 3.
Basically any component should benefit from better power cord. The question is - Is the rest of your system good enough to realize the change?
Upgrade power cords(although not all of them) do improve the sound, but the improvement is much smaller than what you would get from IC or speaker cable upgrade.

I think in your case(with a receiver) it's the source that should benefit from a better power cord.
One power cord that I found to work equally good and make a noticable improvement for both digital and analog components is Audience powerChord. It is unshielded, but an excellent cord for either source or the amp. Good luck.
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If all your gear uses the same power cord you could just keep swapping until you found the best place for it. Or buy two different cables and do the same.
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1. add a dedicated line first.

2. Upgrade the electrical outlet.

3. Now upgrade your power cord.
My experience: The source. I use Harmonix Studio Masters. Dedicated line and power conditioner, as well. All for the source. peace, warren :)
Tvad, $5 cord? that's cool, but I said "basically". And the better power cord in your case was the $5 cord for that particular component.
It's a combination of things. Not necessarily one upgraded cord is going to make a huge improvement. Have you thought about what the rest of the power cords in your system are doing? Is it possible that all the other cords there in your system did more harm than good in the combination with your pre-amp and then you drop this el-cheapo cord that in the context of another system would probably sound like crap. And all of a sudden things come to life?
I appreciate what you're saying, but the most important thing is system synergy. You achieved it with the addition of that $5 cord. Congrats. :)
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I always put the PC on the cdp first....amp thereafter but amp pc effect was subtle and the cdp was not.

Tvad, your suggestion comes right on time. I need a cheap cord now while I am waiting for my Audience cords to arrive. I have all the stock cords burried somewhere in the closet and I'm too lazy to dig in there for them.
I may consider the Volex.
I would agree with the earlier post of having a dedicated line first, then outlets, and then power cords.

I did it that way, and the dedicated circuits were the most dramatic improvement of anything.

I have a dedicated 20A circuit for my amps and one for my front end.
Thanks for everyone's thoughts. My system consists of:

Denon DVD2200 (DVD, CD/SACD)
NAD T763
Spendor S5e
I am running direct 5.1 from the Denon with Audio Note AN-S silver interconnects
Speaker cable is Luminous Renaissance

Obviously, my system is a little far from high-end, so perhaps a PC upgrade will have little impact at this point?
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Tvad is dead on; power cords would be diminishing returns at the point you are at. Definitely save for component upgrades - much bigger bang for buck over time.