I would say it could be read a couple of ways. And the fact that it is Septuagint Greek marks it quite differently from later Koine Greek (i.e. the New Testament). LXX Greek in many was was the bridge to gap Classical and N.T. Greek. My 4th year prof was always keen on completely ignoring [face=SPIonic]de[/face] when we came upon it in Classical settings. He was of the opinion that it was mainly used for punctuation (much like a period...i.e. a new sentence starts here) and could be left out when we were translating. He would probably prefer just "I am poor and needy" as a translation.
On the other hand....it seems pretty clear to me that in the context of the passage the [face=SPIonic]de[/face] is acting like a signpost, drawing a contrast to the previous statement. What the Psalmist is saying is, "Those that seek you rejoice k.t.l., but HEY....what about me God?" Therefore, I would be most tempted to tranlate it as "But I am poor and needy".
Just my two denarii worth