I'm not so sure on the removal of the 2 512MB sticks. If your motherboard has 4 DIMM slots and is capable of operating in dual channel you should be able to fit all 4 sticks with no loss of performance. This, of course, depends on the speed and timings of the RAM you're adding and the RAM that's currently installed. Say you have 4 channels called A1, A2, B1 and B2. When sticks are installed in A1 and A2 those 2 sticks operate in dual channel. When sticks are installed in B1 and B2 those 2 sticks also operate in dual channel. If there is a mismatch in speeds or timings the system will revert to the slowest settings, but you may find that this slight decrease is outweighed by the increase in available RAM, depending on exactly what you use the PC for. The only way for you to find out for sure is to try each configuration for a period of time, performing normal tasks and see if you notice a difference.
Now back to the upgrades. If you were to install a PCI-E graphics card to replace the onboard graphics you would see a significant increase in performance in LFS and other graphics intensive tasks. However, if you plan to get a new PC in 7 months time you may not want to invest much more money in this PC. If the PC you're going to be getting in 7 months time also has integrated graphics with expansion for PCI-E you could carry over a new card you buy for your current PC. This is something you'll need to research and think about. A reasonable low profile PCI-E card will cost you about £50.