1. RAM - upgrade to 4GB - this is the cheapest/easiest solution.
2. Video card - pop open your computer case to see if you have an AGP or PCI Express slot for a video card - it may already be populated with a video card now. If it has one, then depending on the slot type you can upgrade to a number of better video cards that can handle STO - depending on if/what video card you can get, this can significantly improve your performance.
3. Processor - you'll have to do some homework to see what your Dell can handle, but you should be able to upgrade your processor to a Core 2 Duo of some flavour. Ebay is a good source for locating a cheap used processor - this will also make a significant improvement in not just STO, but your whole computer experience.
4. Your Dell probably comes with a SATA port or 2, so finally you can upgrade your Hard Drive to an SSD - this would be your last and final upgrade, as it wouldn't really affect the performance of STO (except for loading the game at the start).