Woodsong, as someone asked above, you say you've been clean for a week - what from? If you just kicked benzos, and this was a relapse, then you might be at some risk of aggravated withdrawal once the Xanax is out of your system. In that case, I'd say get hold of a little valium, and either do a quick taper, or just keep a 10mg pill or two on you in case of sudden physical withdrawal symptoms (tremors, for example). If you just kicked something else, it's probably unlikely that you're in for anything worse than feeling shitty and spaced for a few days: but benzos are tricky, and you might be best off keeping a SMALL supply of valium on hand to deal with any sudden, harsh rebound effects.
As other posters have said, this is a learning experience: you got behind the wheel on 40mgs of alprazolam, you're lucky to be alive, free - and not to have killed anyone. It sounds like maybe you ought to swear off short-acting benzos altogether once this current crisis has passed, and stay off - I'm not moralising, I'm tapering from a high-dose Xanax habit on Valium now, and about six months ago, took 450mgs of diaz in the course of 24 hours, sleeping in three 8-hour shifts after a bad night and race to a tight deadline - but Xanax is evil for amnesia in a lot of people, and compulsive redosing can turn a Catholic schoolgirl with a license and small auto into a serial killer (I've never blacked out on anything but lorazepam and booze, but I've never taken more than 4mgs of Alprazolam at a time, though I did go through a period of dosing 4mgs 3 times a day). If you can't control your dosing, you shouldn't dose at all or you could wake up in a cell facing charges of vehicular manslaughter, DUI and destruction of property. Benzos and driving really don't mix. I'm not being harsh, just pointing out, as Erowid does, that driving while intoxicated is taking risks with other people's lives: way I see it, we're all free to risk our own lives as we choose, but putting others in danger is a whole different story. You could end up in serious trouble, ruin lives and leave a family or two with permanent grief trauma and for what? A benzo 'high'? There isn't one. That's why they're called downers.
I think part of the reason so many of us get into trouble with benzos is that we overdose, chasing a buzz - and there isn't much of one. They're useful for short-term treatment of panic attacks, aborting bad trips and stimulant comedowns, and in treatment of epilepsy when other anti-convulsants fail, severe alcohol withdrawal and PTSD, but recreational use is pretty lame: everyone's different, of course, and Xanax is one of the more pleasant benzos (if you're not too blacked out to know you're high), but IME, only really euphoric when used to potentiate opiates, and then you're risking death by respiratory depression. They're not MDMA, or even Vicodin, just chill pills - and less is generally more, as your experience demonstrates, dramatically.
In the short-term, though, if you've just relapsed a week after kicking a benzo habit, then you're probably going to need, paradoxically, more benzos to get through it. Valium is your friend, here - Xanax is too short-acting to taper off of. So if it was a benzo habit you just kicked, rebound withdrawals could become serious - read the Ashton manual and either get hold of a supply of diazepam and decide a taper schedule, or, ideally, do so under medical supervision. A little research, depending on where you are, will probably turn up a benzo recovery specialist. In the short term, if you're going to need treatment, get hold of some valium - or if you can't, whatever benzo you can - to have on hand in case of sudden crisis. Any symptoms more severe than mild anxiety & fuzziness and you should probably check in with a doctor.
Even if you just kicked something else, if you can get hold of a tab or two of valium, and trust yourself not to just swallow them for the hell of it, it might not hurt to have them on hand in case of sudden shakiness or panic attacks as the Xanax leaves your system: just one or two 10mg doses (diazepam, NOT xanax) would get you through the worst of it, and if you stopped there, be pretty much self-tapering. But that's just being extra cautious: in all likelihood, you're just going to feel foggy, forgetful, with the lethargy and possibly nausea of a bad booze hangover, without the pounding headache. Drink lots of water to help eliminate the residue, and at the risk of sounding preachy, as someone in treatment for a benzo habit that started with xanax, benzos really are a zero-sum game when it comes to regular use. They give a lot of relief...and then take back, with interest, like one of those shitty credit cards-for-poor credit.
Glad you made it through alive and without blood on your hands: be careful over the next few days, and avoid driving if you can. If you must drive, a little caffeine and keeping your eyes & thoughts focused squarely on the road seem like a good idea. Take it easy, man, and good luck with the aftermath. Shit, 40mgs of alprazolam = 800mgs of diazepam. 80 big blues...if you'f thrown in an OC80 or a little dope, you'd be on a slab somewhere. Fucking benzos, man: shitty high, limitless capacity to do damage. Like Voting Nader in Florida 12 years ago...