I guess I should weigh in as the resident Jew here.
Cash in any denomination is welcome. While at my Bar Mitzvah (mumbledy-mumble years ago) I received some gifts in multiples of 18 (more on that later), but most were round numbers. $50, $100, $200. (My neighborhood was fairly well-to-do.) Merchandise gifts are usually rare, but sometimes seen. The traditional merchandise gift used to be a fountain pen. (Why? I'm not sure. It originated before my time, but the traditional Bar Mitzvah speech joke would be to begin with "Today, I am a fountain pen.")
The story with gifts in multiples of $18:
The Hebrew word for life is "chai", which consists of the letters chet and yud. (Note: those use the hard "ch" sound.) Hebrew numbers are also sometimes written using the letter characters. Chet (8) and yud (10) are "18". So, by giving a gift of 18, you are giving a gift of life. While we frequently do this for life cycle events (most recently my brother's wedding), I have rarely, if ever, seen it done by non-Jews.
Just give what you would normally give, and it'll be fine.
Of course, I'll temper the above advice along with the "all men must wear yarmulkes" and women's dress code advice from mwitkus by saying that every congregation is different and has its own rules. If it's an Orthodox congregation, then you might not even be able to sit with your spouse, as they segregate the sexes. If it's Conservative, you'll probably still have all men in yarmulkes and women will be conservatively dressed. Reform (NOT "reformed" -- it's an ongoing process) congregations tend to be much more liberal, and vary on the yarmulke requirements, though some congregations even request men not to wear them. Best bet on attire, if you're not sure, is to doublecheck with the inviting family. They'll (most likely) be glad that you are concerned about propriety.
As far as a gift goes, don't worry. Just round it off and give what you plan to give. If you want to round it off to a multiple of chai, go ahead, and you could even mention in the card something like "here's three chai to help celebrate this day" or something like that. I usually find some way to link the number of chai that I'm giving to the person or the event. Not always easy. Either way, don't sweat it.
I hope I didn't go on too long, and that this was helpful. If you've got any more questions, please ask.
Most importantly of all: Have fun!