From my experience 17" wheels seem to be an optimum size in comfort/handling/price ratio.
3. 20mm Impreza STi rear sway bar, arguably Ze biggest bang for the buck along with the tires.
Once you've covered these basics you can start looking at lowering your Forester. There are quite a few options here - you can change springs, or springs+struts, you can get coilovers. At least for 04/05 model year Foresters the full 04 Impreza STi suspension was a direct swap. Some people that prefer the best medium between comfort and handling install JDM Forester STi pink springs. They lower the car for about 1" and a bit stiffer but not stiff in comparison with the STi springs. Also there are people that install swift springs. Search this board for them and you should find a few interesting threads with details.
16G powered 04 FXT 5MT-Sold
07 Impreza STI-Current