In that case, I'd go with a 3 way pan and tilt head that you can lock out to hold your kit rock steady.

I'm not sure whether the tripod you've got now would facilitate changing the head, but even if it did I would go for something different as the leg brace on it means that you cannot get really close to ground level which you'd need to do for plant / insect close ups.

I'm in no way connected to Manfrotto but use their tripod (MT294A3) and head (804RC2) which I got in the UK for just under £100 last December.

I have a D90 which I use with a 18-105 lens or a Tamron 10-24, and once tightened down there is no movement. One down side is that it isn't the lightest to carry around.

Oh, and the man in the shop reckoned it'd last longer than me!!!