The more RAM you have on the card the less hesitant it tends to be in graphic heavy previews, and bigger textures. If you're using D|S 3 it doesn't hurt to go with a 512MB or 1GB card in the 7,8 or 9000 nVidia series chips. If you're not playing video games I would think dropping more than $100 US would be frivolous. 3Delight does not use your video card when it renders so the only advantage to the video card is when you're previewing on screen. Most software rendering engines, 3Delight in perticular use only CPU speed and system RAM and ignore the video card.

Just so you know nVidia doesn't make those cards, they make the chipsets and sell to vendors like ASUS, MSI, PNY, Galaxy, etc. So it almost doesn't matter who's you buy unless you have a personal preference. I tend to like ASUS but their support can be terrible, the cards are well made.

Just make sure you have enough space in you computer box and that the connectors on the old card match the ones on the new card (They should, this should not be a problem but it might be) You may have to run power to the card to get it to work, you may not. 

Are you going to have someone connect it for you? If not I can probably walk you though it. If you plan on opening the computer up it makes sense to buy a can of compressed air to blow those dust bunnies out too, or in the case of a 3 year old system possibly dust lions.