What do I need to upgrade and what are the cheapest to upgrade?

Okay I'm a Sims 2 fanatic. I own The Sims 2 ofcourse, Pets, Seasons, University, Nightlife and thinking about adding to my collections. I have a sucky computer but AMAZINGLY..everything runs smooth having deleted almost every inch of software in my computer. I set all my options on my graphics card to preformance and it runs incredibly even when I have every option in the game set on high. EVERYTHING in my computer runs fast ever since I did work on it..Only problem is...well..the quality. It damn sucks really bad..I have some texture errors in the game Long loading time..And special events camera wont turn on because it grayed out. The sky in the sims 2 looks like it has smoothed diamond textures instead of it being clear smooth and i cant see the moon or sun...sometimes I can see stars...but not like im supposed to... also...when the textures are set on high...they sometimes switch to medium even when still on high and my sims cloths get all washed out and you cant see the detail...same thing with pictures and pets....and the pool doesnt have ripples like its supposed to....I can only invite 8 guest to a wedding party...and 5 to a house and sports party...And whenever I go to a lot...there is only like...3 other people on that lot besides my family.....thats not even the half of it...but I'll stop there...I know that my card is outdated...but is there any way I can make anything better without pimping it with new hardware...or even with a new computer....considering these specs im about to give..this is the fastest computer out there lol Specs: Dell Dimension 4600; 256 mb RAM; 40GB hardrive; Microsoft Windows XP Professional; Intel Pentium 4 CPU 2.40GHz; Processor Speed: 2.34 GHz; NVIDIA GeForce MX 440 AGP8X 64mb RAM..what should I upgrade to that will make better gaming quality...that is inexpensive...Im only 14...I have to bribe my parents so not too expensive!