Via chipsets aren't bad in any way (they used to have issues long ago), but for the Athlon XP nvidia is simply better.
AGP is end-of-life. AGP cards are still being released now and then with very powerful chips and i don't think that for gaming it is even that much slower, but it makes very little sense to invest in a technology that is already a legacy. They're also more expensive and hard to obtain compared to a similarly performing PCI-E card.
PCI-Express has the benefit of being a universal and scalable technology, thus cheaper to implement, it has more bandwidth, it is more flexible, and it can supply more power. For instance things like SLI or using your GPU as a coprocessor are (almost) impossible to do over AGP.
I strongly suggest you consider offering your current graphics card up for sale on Ebay or whatever and getting a pci-express model with a pci-express mobo. For the rest it's a matter of comparing prices and reviews, but the Intel Core2 series holds the undisputed performance and overclocking crown, though AMD is still competetive in price/performance. This also means you will have to upgrade your memory to DDR-2 and if you have more two IDE devices you may have to get some ide-sata converters as modern Intel boards have only one IDE port.
Its just a bit of a difficult time for easy upgrades ATM.
I do see a cheaper alternative, you can upgrade to older hardware. An old fashioned Athlon 64 in a motherboard with Socket 939, an AGP slot and DDR memory. That'll be cheap, especially if you get second hand kit, you can reuse graphics and memory, you can even get dual core i believe, but you won't have a future upgrade path.
For motherboards, i prefer Asus as they've never let me down, but these days MSI and Abit are good also, as well as other brands. Read reviews before you buy!
