With a mini tower I suppose you mean a small form tower rather than a real mini tower. What's the difference, you might ask? Regular tower formed computer cases can be categorized as mini-tower, mid-tower or full-tower, all of which contain one or more horizontal 5" slots for optical drives and such. Their main difference is height, although some can also be much wider than others for better cable management and breathing.
There's more to take into consideration than just the size when upgrading a video card. I suppose your entire rig is about the same age than the X1300. A significantly more powerful video card would cause bottlenecks in more than one place in a ten year old small factor computer. More important, your power supply won't be powerful enough to feed the new card. The X1300 was a very low power card, only 30 W. Sad to say, but you'd better get a new computer instead of just upgrading the video card, because the card won't be the only new component you'd need.
In case your budget is VERY tight, which I of course can't tell by the information you've given so far, here's some ghetto mod/Frankenstein suggestions: People dump XP rigs now that the OS isn't supported any more. An abandoned PC is where you'd get a case for free. With some luck there's an upgraded power supply, so you'd only have to find a video card. There's a couple decent ones on the market that take all the power they need through the pci-e slot, making them more suitable for older computers.
The more expensive puzzle would be getting a new case, PSU and video card and use the rest of your current rig until something else needs replacing. The next big update is already being coded, so the upgrading time can be quite near.
