I'm not really sure about the processor. What kind is it? As for the video card, do you know what kind of slot it uses? I'm thinking that it would be AGP, but I'm not sure. If it is then
something like this should be a nice upgrade, but you might need a new power supply to go with it.
BTW, wait until other people give you some suggestions too, I've been know to not know what the hell I'm talking about.
Edit: It might be worth pricing and building a newer computer yourself though. Now that I think about it you would pretty much have to replace everything to upgrade this one. The motherboard probably would have to be replaced to use a modern processor, so that pretty much means that the ram and video card would have to go. The only things that I can see not replacing would be the drives, sound card, and modem.
Edit 2: I put together a computer that should run TS2 nicely, and not be crazy expensive to build. If you can get your fiancé to build it for you, then this would be cheaper than getting it from a company like Dell or HP.
Processor - Intel Pentium E2220 Allendale 2.4GHz $89.99
Memory - OCZ S.O.E 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 - $26.99 after mail in rebate.
Motherboard - GIGABYTE GA-G31M-S2L LGA 775 Intel G31 Micro ATX Intel Motherboard - $59.99
Power Supply - Antec True Power Trio TP3-550 550W ATX12V - $59.99
Video Card - XFX PVT84JYAJG GeForce 8600 GT 512MB - $62.99 after mail in rebate.
DVD Drive - SAMSUNG 22X DVD±R DVD Burner with LightScribe - $30.99
Hard Drive - Western Digital Caviar SE16 WD3200AAKS 320GB 7200 RPM SATA - $64.99
Case - COOLER MASTER Centurion 541 RC-541-SKN1 Black AluminumOperating System - Windows XP Home SP3 - $84.99
Case Fans - MASSCOOL FD08025S1M4 80mm - $5.37 for 3
This would make a great TS2 computer, or a computer for anything else really. It has enough ram and a fat processor, plus a great video card. You would easily be able to run The Sims 2 all the way on high, and TS3 should be nice on it too. In the future if you need to replace anything it should be easy to upgrade. If you shop around for the parts you should be able to get several of them cheaper.