Are those 320k unique visitors per month, or are you taking the unique daily visitors and adding them together? Because if you're getting 320k unique visitors, then you could be making some serious advertising revenue.
Here's a survey of average CPM rates in 2009 for vertical advertising. I don't know if these are banner ads or textual ads, but they seem to range from $1-2 to $15-17. Luckily for you, for the tech industry, the average is around $15, which is at the high end.
CPM rates are calculated per thousand impressions. So 5 million page views would be $15 * 5000 = $75K. That's a nice chunk of change you have there if the advertiser is U.S.-based and has this sort of marketing budget.
That said, different publishers will be able to realize different CPM rates. I've seen people give $30-35 as sample CPM rates for banner ads, but most publishers probably won't be able to ask for that much. I would start at $15 and negotiate from there.
And if the advertiser doesn't think your ad space is worth that much, but you think it's worth more than what they're offering, you could propose a CPA/CPC deal where they pay per action/conversion. This way, there's zero risk to the advertiser, and you can ask for a higher rate than they're willing to offer for CPM. Alternatively, you could also use a PPC arrangement, which would be somewhere in between CPM and CPA.