Hi, HookahRoo. Welcome to the forum.
I would look at the stickied thread here on video cards (
Graphics Card). It has some advice on choosing a graphics card for The Sims 3.
I would caution about spending too much to upgrade the Optiplex 745 into a "gaming" system however. We have several Op 745 systems where I work and from my experience trying to make a "business" class PC into a "gaming" PC is more expensive than it's worth. The reason is you'll end up needing to replace the graphics card and the power supply at the very least. The only way to update the processor and the memory (beyond 4 GB) is to replace the motherboard. Depending on the case you have (Small Form Factor, Desktop, Mini-Tower) you may have to find half-height cards which are generally harder to find and a little more expensive.
My suggestion is to budget for a replacement system. You can get a decent desktop system in the $600 - $1,000 (US) range, and you won't have to spend a lot for extra parts. The closer you get to $1,000 the less "extras" you'll need to get to make it run Sims 3 well.
Hope this helps.
Flynn