If you're not living in a house where you're adding lots of gadgets like computers, DVD machines, and the like, and/or in a house where everything like heating and cooking and hot water is run off electricity, your current service may be just fine.
However, if you are living the aforementioned type of life (especially with children or a gadget-hungry spouse), it may be worth considering an upgrade. As to whether or not 100 amp service is sufficient, your electrical contractor will discuss that with you. I live in SE PA and 200 amp service upgrade from 60 amp ran between $1200 and $1500 3 years ago. I'd be inclined to think that the high side would apply today.
In the mean time, just for comparative purposes, there isn't anything that should prevent you from adding a separate breaker for an EXTERIOR electrical outlet that works off of an IN-THE-HOUSE switch (so you no longer have to run in and out to turn the lights on and off every night.) When we had our service upgraded, we had the electrician put in a split GFCI outlet where the top-half is ALWAYS 'hot' and the bottom-half works off a switch inside the house (for just your type of purpose!) It's great to be able to just flick a switch in the comfort of your home!
I would guess that a licensed electrician would charge about $250, give or take, to give you such a set-up. What you might want to consider is having a friend or family member put you in touch with someone looking for some 'side work' in the trades. It will probably cost a little less AND they still know what they're doing. The BIGGER expense of resizing your service may not even be necessary but, again, have a licensed contractor or 3 give you written estimates. You DON'T have to have the work done to have someone come to your home to give an estimate. Usually, the estimate consists of the following in your case: Upgraded breaker box, new breakers, new service line, new meter, and labor. This is 'in general' so just ask your contractor if it would apply to you.
One last thought ... Just because you got an upgrade to your service or just because you have a DEDICATED circuit for your lights doesn't mean you can 'go crazy' with lights. All it means is that the DEDICATED circuit can handle XYZ amps; that's all. In addition, you need to be sure that as you string your decorations and light to the outlet, you need to be sure the decoration wiring, itself, can handle the load as you put more and more 'load' on the line. This is why it's important to read the boxes and decorations to see how many 'amps' the line can carry at a maximum. Otherwise, you'll melt the wiring on your decorations! (Just a heads-up.) If necessary, have the electrician put BOTH sides of the EXTERIOR GFCI on the switch. You will still only be able to run XYZ amps TOTAL but you can break up your 'last connection' into 2 connections. This is how you can lighten up the load on some of your decoration's wiring. My benchmark is: If you're running anywhere over 10 amps on 1 circuit for Holiday Lights, have another circuit installed (with or w/o its own switch.)