While you might think that you have a fair gap between your flights, it's actually not going to give you much time to go shopping. Having regularly travelled to Singapore & stayed in Orchard Road when there, it normally took about 90 minutes from stepping off the plane to climbing up out of the MRT station under the Marriott at Tangs Plaza in Orchard Road.
I used to find that taking a taxi was pretty much of a gamble as far as time was concerned to get from the airport into Orchard Road. One time if the traffic was light & no hold ups, or queue at the airport taxi rank, you could expect to be at the hotel in less than an hour from the plane, other times, if you were unlucky & arrived after a few 747's etc. landed, there could be a queue for immigration, bags and taxis that would eat up a good 15-20 minutes for each. Add to that Singapore traffic at a busy time, the odd accident & roadworks and you can be looking at well over an hour in the taxi.
What time you take going in, is likely to be the same coming back, so although you have just under six hours on the ground, this could be eaten up just landing & getting to & from Orchard Road.
Even going by the MRT direct from the terminal, which doesn't suffer any jams and runs to an incredibly precise timetable, you still won't have much time to spare, as you will likely need to be back at the gate to go through gate security at Changi at least an hour before boarding your next flight. :-(. This is also in addition to the initial security screening & passport control just to get airside, which again, although quick, can take time.
I really don't want to rain on your parade, but I would estimate that you could really only expect to have about 90 minutes (safely) shopping during your stopover which is completely inadequate to do justice to Orchard Road, never mind the whole of Singapore.
I'd suggest that you just take in the airport on your first transit, as it's got a whole load of shops, restaurants etc. to keep you amused without running the risk of missing your connecting flight for the sake of a minor saving.
I'd also caution that electronics are no longer a major bargain in Singapore, prices are only a little cheaper for some things and can be more expensive for others (the last couple of times I was there it was slightly cheaper to buy I-Pads & I-Pods in the UK than there). The best place for electronics is probably still Sim Lim Square, where the higher you go, the better the prices can become, but it's not the place to be considering when you are pressed for time, and it can be a bit hit or miss whether they have what you are specifically looking for or not.
Have fun and enjoy your visit whatever you decide to do, but save the shopping for the return leg when you have plenty of time.