What subject are you considering doing a Ph.D. in? If it is even vaguely sciencey, you should be able to get a full tuition waiver and a stipend at any reputable program so that you won't have to work part-time. Ph.D's take long enough as is - there's really not a good reason to drag out the process if you can avoid it.
I would also note that although it varies from field to field a bit, only a little over 30% of people getting science Ph.D's in 2008 ended up in tenure-track positions. This makes the Ph.D. game a bit of a crapshoot. It is very easy and plausible to end up as a "beltway flier", adjuncting at three or four different institutions at the same time, cobbling together four or five classes a semester to make ends meet.
Not saying that Ph.D's are never worth it, just saying that there is definitely a lot of risk involved. Some Ph.D's (like engineering or chemistry) lead into jobs in industry and are safer bets, but the more esoteric ones have rather fraught prospects.