I would buy and install the 256MB RAM first, and try it out. Spending max £100 on a graphic card won't get you that much improvement over your TI4200 in my view.
You'll get a newer card of course, with support for newer features, but not that much boost to speed i think.

You'll need to spend around £150 to really get a graphic card that will outshine your 4200 enough for it to be worth the money (again, in my opinion).
Then again, i haven't checked pricelevels for a couple of months (since i bought my new PC)... maybe there's some new fast budgetcards in the £100 range that i don't know about.

The added RAM might very well increase your performance in RTW a lot though! With only 256MB your PC probably runs out of memory in intensive situations, and when that happens it uses your HD instead (virtual memory) and that's MUCH slower.